3. The SMTP Procedures: An Overview This section contains descriptions of the procedures used in SMTP: session initiation, mail transaction, forwarding mail, verifying mailbox names and expanding mailing lists, and opening and closing exchanges. Comments on relaying, a note on mail domains, and a discussion of changing roles are included at the end of this section. Several complete scenarios are presented in Appendix D.
3.1. Session Initiation An SMTP session is initiated when a client opens a connection to a server and the server responds with an opening message. SMTP server implementations MAY include identification of their software and version information in the connection greeting reply after the 220 code, a practice that permits more efficient isolation and repair of any problems. Implementations MAY make provision for SMTP servers to disable the software and version announcement where it causes security concerns. While some systems also identify their contact point for mail problems, this is not a substitute for maintaining the required "postmaster" address (see Section 4). The SMTP protocol allows a server to formally reject a mail session while still allowing the initial connection as follows: a 554 response MAY be given in the initial connection opening message instead of the 220. A server taking this approach MUST still wait for the client to send a QUIT (see Section 126.96.36.199) before closing the connection and SHOULD respond to any intervening commands with "503 bad sequence of commands". Since an attempt to make an SMTP connection to such a system is probably in error, a server returning a 554 response on connection opening SHOULD provide enough information in the reply text to facilitate debugging of the sending system. 3.2. Client Initiation Once the server has sent the greeting (welcoming) message and the client has received it, the client normally sends the EHLO command to the server, indicating the client's identity. In addition to opening the session, use of EHLO indicates that the client is able to process service extensions and requests that the server provide a list of the extensions it supports. Older SMTP systems that are unable to support service extensions, and contemporary clients that do not require service extensions in the mail session being initiated, MAY use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- style response to a HELO command. For a particular connection attempt, if the server returns a "command not recognized" response to EHLO, the client SHOULD be able to fall back and send HELO. In the EHLO command, the host sending the command identifies itself; the command may be interpreted as saying "Hello, I am <domain>" (and, in the case of EHLO, "and I support service extension requests").
3.3. Mail Transactions There are three steps to SMTP mail transactions. The transaction starts with a MAIL command that gives the sender identification. (In general, the MAIL command may be sent only when no mail transaction is in progress; see Section 4.1.4.) A series of one or more RCPT commands follows, giving the receiver information. Then, a DATA command initiates transfer of the mail data and is terminated by the "end of mail" data indicator, which also confirms the transaction. The first step in the procedure is the MAIL command. MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF> This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. The <reverse-path> portion of the first or only argument contains the source mailbox (between "<" and ">" brackets), which can be used to report errors (see Section 4.2 for a discussion of error reporting). If accepted, the SMTP server returns a "250 OK" reply. If the mailbox specification is not acceptable for some reason, the server MUST return a reply indicating whether the failure is permanent (i.e., will occur again if the client tries to send the same address again) or temporary (i.e., the address might be accepted if the client tries again later). Despite the apparent scope of this requirement, there are circumstances in which the acceptability of the reverse-path may not be determined until one or more forward-paths (in RCPT commands) can be examined. In those cases, the server MAY reasonably accept the reverse-path (with a 250 reply) and then report problems after the forward-paths are received and examined. Normally, failures produce 550 or 553 replies. Historically, the <reverse-path> was permitted to contain more than just a mailbox; however, contemporary systems SHOULD NOT use source routing (see Appendix C). The optional <mail-parameters> are associated with negotiated SMTP service extensions (see Section 2.2). The second step in the procedure is the RCPT command. This step of the procedure can be repeated any number of times. RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF> The first or only argument to this command includes a forward-path (normally a mailbox and domain, always surrounded by "<" and ">" brackets) identifying one recipient. If accepted, the SMTP server returns a "250 OK" reply and stores the forward-path. If the
recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible). The <forward-path> can contain more than just a mailbox. Historically, the <forward-path> was permitted to contain a source routing list of hosts and the destination mailbox; however, contemporary SMTP clients SHOULD NOT utilize source routes (see Appendix C). Servers MUST be prepared to encounter a list of source routes in the forward-path, but they SHOULD ignore the routes or MAY decline to support the relaying they imply. Similarly, servers MAY decline to accept mail that is destined for other hosts or systems. These restrictions make a server useless as a relay for clients that do not support full SMTP functionality. Consequently, restricted- capability clients MUST NOT assume that any SMTP server on the Internet can be used as their mail processing (relaying) site. If a RCPT command appears without a previous MAIL command, the server MUST return a 503 "Bad sequence of commands" response. The optional <rcpt-parameters> are associated with negotiated SMTP service extensions (see Section 2.2). Since it has been a common source of errors, it is worth noting that spaces are not permitted on either side of the colon following FROM in the MAIL command or TO in the RCPT command. The syntax is exactly as given above. The third step in the procedure is the DATA command (or some alternative specified in a service extension). DATA <CRLF> If accepted, the SMTP server returns a 354 Intermediate reply and considers all succeeding lines up to but not including the end of mail data indicator to be the message text. When the end of text is successfully received and stored, the SMTP-receiver sends a "250 OK" reply. Since the mail data is sent on the transmission channel, the end of mail data must be indicated so that the command and reply dialog can be resumed. SMTP indicates the end of the mail data by sending a line containing only a "." (period or full stop). A transparency procedure is used to prevent this from interfering with the user's text (see Section 4.5.2). The end of mail data indicator also confirms the mail transaction and tells the SMTP server to now process the stored recipients and mail
data. If accepted, the SMTP server returns a "250 OK" reply. The DATA command can fail at only two points in the protocol exchange: If there was no MAIL, or no RCPT, command, or all such commands were rejected, the server MAY return a "command out of sequence" (503) or "no valid recipients" (554) reply in response to the DATA command. If one of those replies (or any other 5yz reply) is received, the client MUST NOT send the message data; more generally, message data MUST NOT be sent unless a 354 reply is received. If the verb is initially accepted and the 354 reply issued, the DATA command should fail only if the mail transaction was incomplete (for example, no recipients), if resources were unavailable (including, of course, the server unexpectedly becoming unavailable), or if the server determines that the message should be rejected for policy or other reasons. However, in practice, some servers do not perform recipient verification until after the message text is received. These servers SHOULD treat a failure for one or more recipients as a "subsequent failure" and return a mail message as discussed in Section 6 and, in particular, in Section 6.1. Using a "550 mailbox not found" (or equivalent) reply code after the data are accepted makes it difficult or impossible for the client to determine which recipients failed. When the RFC 822 format (, ) is being used, the mail data include the header fields such as those named Date, Subject, To, Cc, and From. Server SMTP systems SHOULD NOT reject messages based on perceived defects in the RFC 822 or MIME (RFC 2045 ) message header section or message body. In particular, they MUST NOT reject messages in which the numbers of Resent-header fields do not match or Resent-to appears without Resent-from and/or Resent-date. Mail transaction commands MUST be used in the order discussed above. 3.4. Forwarding for Address Correction or Updating Forwarding support is most often required to consolidate and simplify addresses within, or relative to, some enterprise and less frequently to establish addresses to link a person's prior address with a current one. Silent forwarding of messages (without server notification to the sender), for security or non-disclosure purposes, is common in the contemporary Internet. In both the enterprise and the "new address" cases, information hiding (and sometimes security) considerations argue against exposure of the "final" address through the SMTP protocol as a side effect of the forwarding activity. This may be especially important when the
final address may not even be reachable by the sender. Consequently, the "forwarding" mechanisms described in Section 3.2 of RFC 821, and especially the 251 (corrected destination) and 551 reply codes from RCPT must be evaluated carefully by implementers and, when they are available, by those configuring systems (see also Section 7.4). In particular: o Servers MAY forward messages when they are aware of an address change. When they do so, they MAY either provide address-updating information with a 251 code, or may forward "silently" and return a 250 code. However, if a 251 code is used, they MUST NOT assume that the client will actually update address information or even return that information to the user. Alternately, o Servers MAY reject messages or return them as non-deliverable when they cannot be delivered precisely as addressed. When they do so, they MAY either provide address-updating information with a 551 code, or may reject the message as undeliverable with a 550 code and no address-specific information. However, if a 551 code is used, they MUST NOT assume that the client will actually update address information or even return that information to the user. SMTP server implementations that support the 251 and/or 551 reply codes SHOULD provide configuration mechanisms so that sites that conclude that they would undesirably disclose information can disable or restrict their use. 3.5. Commands for Debugging Addresses 3.5.1. Overview SMTP provides commands to verify a user name or obtain the content of a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. Implementations SHOULD support VRFY and EXPN (however, see Section 3.5.2 and Section 7.3). For the VRFY command, the string is a user name or a user name and domain (see below). If a normal (i.e., 250) response is returned, the response MAY include the full name of the user and MUST include the mailbox of the user. It MUST be in either of the following forms: User Name <local-part@domain> local-part@domain
When a name that is the argument to VRFY could identify more than one mailbox, the server MAY either note the ambiguity or identify the alternatives. In other words, any of the following are legitimate responses to VRFY: 553 User ambiguous or 553- Ambiguous; Possibilities are 553-Joe Smith <firstname.lastname@example.org> 553-Harry Smith <email@example.com> 553 Melvin Smith <firstname.lastname@example.org> or 553-Ambiguous; Possibilities 553- <email@example.com> 553- <firstname.lastname@example.org> 553 <email@example.com> Under normal circumstances, a client receiving a 553 reply would be expected to expose the result to the user. Use of exactly the forms given, and the "user ambiguous" or "ambiguous" keywords, possibly supplemented by extended reply codes, such as those described in RFC 3463 , will facilitate automated translation into other languages as needed. Of course, a client that was highly automated or that was operating in another language than English might choose to try to translate the response to return some other indication to the user than the literal text of the reply, or to take some automated action such as consulting a directory service for additional information before reporting to the user. For the EXPN command, the string identifies a mailing list, and the successful (i.e., 250) multiline response MAY include the full name of the users and MUST give the mailboxes on the mailing list. In some hosts, the distinction between a mailing list and an alias for a single mailbox is a bit fuzzy, since a common data structure may hold both types of entries, and it is possible to have mailing lists containing only one mailbox. If a request is made to apply VRFY to a mailing list, a positive response MAY be given if a message so addressed would be delivered to everyone on the list, otherwise an error SHOULD be reported (e.g., "550 That is a mailing list, not a user" or "252 Unable to verify members of mailing list"). If a request is made to expand a user name, the server MAY return a
positive response consisting of a list containing one name, or an error MAY be reported (e.g., "550 That is a user name, not a mailing list"). In the case of a successful multiline reply (normal for EXPN), exactly one mailbox is to be specified on each line of the reply. The case of an ambiguous request is discussed above. "User name" is a fuzzy term and has been used deliberately. An implementation of the VRFY or EXPN commands MUST include at least recognition of local mailboxes as "user names". However, since current Internet practice often results in a single host handling mail for multiple domains, hosts, especially hosts that provide this functionality, SHOULD accept the "local-part@domain" form as a "user name"; hosts MAY also choose to recognize other strings as "user names". The case of expanding a mailbox list requires a multiline reply, such as: C: EXPN Example-People S: 250-Jon Postel <Postel@isi.edu> S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu> S: 250 Sam Q. Smith <SQSmith@specific.generic.com> or C: EXPN Executive-Washroom-List S: 550 Access Denied to You. The character string arguments of the VRFY and EXPN commands cannot be further restricted due to the variety of implementations of the user name and mailbox list concepts. On some systems, it may be appropriate for the argument of the EXPN command to be a file name for a file containing a mailing list, but again there are a variety of file naming conventions in the Internet. Similarly, historical variations in what is returned by these commands are such that the response SHOULD be interpreted very carefully, if at all, and SHOULD generally only be used for diagnostic purposes. 3.5.2. VRFY Normal Response When normal (2yz or 551) responses are returned from a VRFY or EXPN request, the reply MUST include the <Mailbox> name using a "<local-part@domain>" construction, where "domain" is a fully- qualified domain name. In circumstances exceptional enough to justify violating the intent of this specification, free-form text MAY be returned. In order to facilitate parsing by both computers
and people, addresses SHOULD appear in pointed brackets. When addresses, rather than free-form debugging information, are returned, EXPN and VRFY MUST return only valid domain addresses that are usable in SMTP RCPT commands. Consequently, if an address implies delivery to a program or other system, the mailbox name used to reach that target MUST be given. Paths (explicit source routes) MUST NOT be returned by VRFY or EXPN. Server implementations SHOULD support both VRFY and EXPN. For security reasons, implementations MAY provide local installations a way to disable either or both of these commands through configuration options or the equivalent (see Section 7.3). When these commands are supported, they are not required to work across relays when relaying is supported. Since they were both optional in RFC 821, but VRFY was made mandatory in RFC 1123 , if EXPN is supported, it MUST be listed as a service extension in an EHLO response. VRFY MAY be listed as a convenience but, since support for it is required, SMTP clients are not required to check for its presence on the extension list before using it. 3.5.3. Meaning of VRFY or EXPN Success Response A server MUST NOT return a 250 code in response to a VRFY or EXPN command unless it has actually verified the address. In particular, a server MUST NOT return 250 if all it has done is to verify that the syntax given is valid. In that case, 502 (Command not implemented) or 500 (Syntax error, command unrecognized) SHOULD be returned. As stated elsewhere, implementation (in the sense of actually validating addresses and returning information) of VRFY and EXPN are strongly recommended. Hence, implementations that return 500 or 502 for VRFY are not in full compliance with this specification. There may be circumstances where an address appears to be valid but cannot reasonably be verified in real time, particularly when a server is acting as a mail exchanger for another server or domain. "Apparent validity", in this case, would normally involve at least syntax checking and might involve verification that any domains specified were ones to which the host expected to be able to relay mail. In these situations, reply code 252 SHOULD be returned. These cases parallel the discussion of RCPT verification in Section 2.1. Similarly, the discussion in Section 3.4 applies to the use of reply codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are recognized but that would be forwarded or rejected were mail received for them. Implementations generally SHOULD be more aggressive about address verification in the case of VRFY than in the case of RCPT, even if it takes a little longer to do so.
3.5.4. Semantics and Applications of EXPN EXPN is often very useful in debugging and understanding problems with mailing lists and multiple-target-address aliases. Some systems have attempted to use source expansion of mailing lists as a means of eliminating duplicates. The propagation of aliasing systems with mail on the Internet for hosts (typically with MX and CNAME DNS records), for mailboxes (various types of local host aliases), and in various proxying arrangements has made it nearly impossible for these strategies to work consistently, and mail systems SHOULD NOT attempt them. 3.6. Relaying and Mail Routing 3.6.1. Source Routes and Relaying In general, the availability of Mail eXchanger records in the domain name system (RFC 1035 , RFC 974 ) makes the use of explicit source routes in the Internet mail system unnecessary. Many historical problems with the interpretation of explicit source routes have made their use undesirable. SMTP clients SHOULD NOT generate explicit source routes except under unusual circumstances. SMTP servers MAY decline to act as mail relays or to accept addresses that specify source routes. When route information is encountered, SMTP servers MAY ignore the route information and simply send to the final destination specified as the last element in the route and SHOULD do so. There has been an invalid practice of using names that do not appear in the DNS as destination names, with the senders counting on the intermediate hosts specified in source routing to resolve any problems. If source routes are stripped, this practice will cause failures. This is one of several reasons why SMTP clients MUST NOT generate invalid source routes or depend on serial resolution of names. When source routes are not used, the process described in RFC 821 for constructing a reverse-path from the forward-path is not applicable and the reverse-path at the time of delivery will simply be the address that appeared in the MAIL command. 3.6.2. Mail eXchange Records and Relaying A relay SMTP server is usually the target of a DNS MX record that designates it, rather than the final delivery system. The relay server may accept or reject the task of relaying the mail in the same way it accepts or rejects mail for a local user. If it accepts the task, it then becomes an SMTP client, establishes a transmission channel to the next SMTP server specified in the DNS (according to the rules in Section 5), and sends it the mail. If it declines to
relay mail to a particular address for policy reasons, a 550 response SHOULD be returned. This specification does not deal with the verification of return paths for use in delivery notifications. Recent work, such as that on SPF  and DKIM  , has been done to provide ways to ascertain that an address is valid or belongs to the person who actually sent the message. A server MAY attempt to verify the return path before using its address for delivery notifications, but methods of doing so are not defined here nor is any particular method recommended at this time. 3.6.3. Message Submission Servers as Relays Many mail-sending clients exist, especially in conjunction with facilities that receive mail via POP3 or IMAP, that have limited capability to support some of the requirements of this specification, such as the ability to queue messages for subsequent delivery attempts. For these clients, it is common practice to make private arrangements to send all messages to a single server for processing and subsequent distribution. SMTP, as specified here, is not ideally suited for this role. A standardized mail submission protocol has been developed that is gradually superseding practices based on SMTP (see RFC 4409 ). In any event, because these arrangements are private and fall outside the scope of this specification, they are not described here. It is important to note that MX records can point to SMTP servers that act as gateways into other environments, not just SMTP relays and final delivery systems; see Sections 3.7 and 5. If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse- path). Formats specified for non-delivery reports by other standards (see, for example, RFC 3461  and RFC 3464 ) SHOULD be used if possible. This notification message must be from the SMTP server at the relay host or the host that first determines that delivery cannot be accomplished. Of course, SMTP servers MUST NOT send notification messages about problems transporting notification messages. One way to prevent loops in error reporting is to specify a null reverse-path in the MAIL command of a notification message. When such a message is transmitted, the reverse-path MUST be set to null (see
Section 4.5.5 for additional discussion). A MAIL command with a null reverse-path appears as follows: MAIL FROM:<> As discussed in Section 6.4, a relay SMTP has no need to inspect or act upon the header section or body of the message data and MUST NOT do so except to add its own "Received:" header field (Section 4.4) and, optionally, to attempt to detect looping in the mail system (see Section 6.3). Of course, this prohibition also applies to any modifications of these header fields or text (see also Section 7.9). 3.7. Mail Gatewaying While the relay function discussed above operates within the Internet SMTP transport service environment, MX records or various forms of explicit routing may require that an intermediate SMTP server perform a translation function between one transport service and another. As discussed in Section 2.3.10, when such a system is at the boundary between two transport service environments, we refer to it as a "gateway" or "gateway SMTP". Gatewaying mail between different mail environments, such as different mail formats and protocols, is complex and does not easily yield to standardization. However, some general requirements may be given for a gateway between the Internet and another mail environment. 3.7.1. Header Fields in Gatewaying Header fields MAY be rewritten when necessary as messages are gatewayed across mail environment boundaries. This may involve inspecting the message body or interpreting the local-part of the destination address in spite of the prohibitions in Section 6.4. Other mail systems gatewayed to the Internet often use a subset of the RFC 822 header section or provide similar functionality with a different syntax, but some of these mail systems do not have an equivalent to the SMTP envelope. Therefore, when a message leaves the Internet environment, it may be necessary to fold the SMTP envelope information into the message header section. A possible solution would be to create new header fields to carry the envelope information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would require changes in mail programs in foreign environments and might risk disclosure of private information (see Section 7.2).
3.7.2. Received Lines in Gatewaying When forwarding a message into or out of the Internet environment, a gateway MUST prepend a Received: line, but it MUST NOT alter in any way a Received: line that is already in the header section. "Received:" header fields of messages originating from other environments may not conform exactly to this specification. However, the most important use of Received: lines is for debugging mail faults, and this debugging can be severely hampered by well-meaning gateways that try to "fix" a Received: line. As another consequence of trace header fields arising in non-SMTP environments, receiving systems MUST NOT reject mail based on the format of a trace header field and SHOULD be extremely robust in the light of unexpected information or formats in those header fields. The gateway SHOULD indicate the environment and protocol in the "via" clauses of Received header field(s) that it supplies. 3.7.3. Addresses in Gatewaying From the Internet side, the gateway SHOULD accept all valid address formats in SMTP commands and in the RFC 822 header section, and all valid RFC 822 messages. Addresses and header fields generated by gateways MUST conform to applicable standards (including this one and RFC 5322 ). Gateways are, of course, subject to the same rules for handling source routes as those described for other SMTP systems in Section 3.3. 3.7.4. Other Header Fields in Gatewaying The gateway MUST ensure that all header fields of a message that it forwards into the Internet mail environment meet the requirements for Internet mail. In particular, all addresses in "From:", "To:", "Cc:", etc., header fields MUST be transformed (if necessary) to satisfy the standard header syntax of RFC 5322 , MUST reference only fully-qualified domain names, and MUST be effective and useful for sending replies. The translation algorithm used to convert mail from the Internet protocols to another environment's protocol SHOULD ensure that error messages from the foreign mail environment are delivered to the reverse-path from the SMTP envelope, not to an address in the "From:", "Sender:", or similar header fields of the message.
3.7.5. Envelopes in Gatewaying Similarly, when forwarding a message from another environment into the Internet, the gateway SHOULD set the envelope return path in accordance with an error message return address, if supplied by the foreign environment. If the foreign environment has no equivalent concept, the gateway must select and use a best approximation, with the message originator's address as the default of last resort. 3.8. Terminating Sessions and Connections An SMTP connection is terminated when the client sends a QUIT command. The server responds with a positive reply code, after which it closes the connection. An SMTP server MUST NOT intentionally close the connection under normal operational circumstances (see Section 7.8) except: o After receiving a QUIT command and responding with a 221 reply. o After detecting the need to shut down the SMTP service and returning a 421 response code. This response code can be issued after the server receives any command or, if necessary, asynchronously from command receipt (on the assumption that the client will receive it after the next command is issued). o After a timeout, as specified in Section 188.8.131.52, occurs waiting for the client to send a command or data. In particular, a server that closes connections in response to commands that are not understood is in violation of this specification. Servers are expected to be tolerant of unknown commands, issuing a 500 reply and awaiting further instructions from the client. An SMTP server that is forcibly shut down via external means SHOULD attempt to send a line containing a 421 response code to the SMTP client before exiting. The SMTP client will normally read the 421 response code after sending its next command. SMTP clients that experience a connection close, reset, or other communications failure due to circumstances not under their control (in violation of the intent of this specification but sometimes unavoidable) SHOULD, to maintain the robustness of the mail system, treat the mail transaction as if a 451 response had been received and act accordingly.
3.9. Mailing Lists and Aliases An SMTP-capable host SHOULD support both the alias and the list models of address expansion for multiple delivery. When a message is delivered or forwarded to each address of an expanded list form, the return address in the envelope ("MAIL FROM:") MUST be changed to be the address of a person or other entity who administers the list. However, in this case, the message header section (RFC 5322 ) MUST be left unchanged; in particular, the "From" field of the header section is unaffected. An important mail facility is a mechanism for multi-destination delivery of a single message, by transforming (or "expanding" or "exploding") a pseudo-mailbox address into a list of destination mailbox addresses. When a message is sent to such a pseudo-mailbox (sometimes called an "exploder"), copies are forwarded or redistributed to each mailbox in the expanded list. Servers SHOULD simply utilize the addresses on the list; application of heuristics or other matching rules to eliminate some addresses, such as that of the originator, is strongly discouraged. We classify such a pseudo- mailbox as an "alias" or a "list", depending upon the expansion rules. 3.9.1. Alias To expand an alias, the recipient mailer simply replaces the pseudo- mailbox address in the envelope with each of the expanded addresses in turn; the rest of the envelope and the message body are left unchanged. The message is then delivered or forwarded to each expanded address. 3.9.2. List A mailing list may be said to operate by "redistribution" rather than by "forwarding". To expand a list, the recipient mailer replaces the pseudo-mailbox address in the envelope with each of the expanded addresses in turn. The return (backward-pointing) address in the envelope is changed so that all error messages generated by the final deliveries will be returned to a list administrator, not to the message originator, who generally has no control over the contents of the list and will typically find error messages annoying. Note that the key difference between handling aliases (Section 3.9.1) and forwarding (this subsection) is the change to the backward-pointing address in this case. When a list constrains its processing to the very limited set of modifications and actions described here, it is attempting to emulate an MTA; such lists can be treated as a continuation in email transit.
There exist mailing lists that perform additional, sometimes extensive, modifications to a message and its envelope. Such mailing lists need to be viewed as full MUAs, which accept a delivery and post a new message. 4. The SMTP Specifications 4.1. SMTP Commands 4.1.1. Command Semantics and Syntax The SMTP commands define the mail transfer or the mail system function requested by the user. SMTP commands are character strings terminated by <CRLF>. The commands themselves are alphabetic characters terminated by <SP> if parameters follow and <CRLF> otherwise. (In the interest of improved interoperability, SMTP receivers SHOULD tolerate trailing white space before the terminating <CRLF>.) The syntax of the local part of a mailbox MUST conform to receiver site conventions and the syntax specified in Section 4.1.2. The SMTP commands are discussed below. The SMTP replies are discussed in Section 4.2. A mail transaction involves several data objects that are communicated as arguments to different commands. The reverse-path is the argument of the MAIL command, the forward-path is the argument of the RCPT command, and the mail data is the argument of the DATA command. These arguments or data objects must be transmitted and held, pending the confirmation communicated by the end of mail data indication that finalizes the transaction. The model for this is that distinct buffers are provided to hold the types of data objects; that is, there is a reverse-path buffer, a forward-path buffer, and a mail data buffer. Specific commands cause information to be appended to a specific buffer, or cause one or more buffers to be cleared. Several commands (RSET, DATA, QUIT) are specified as not permitting parameters. In the absence of specific extensions offered by the server and accepted by the client, clients MUST NOT send such parameters and servers SHOULD reject commands containing them as having invalid syntax. 184.108.40.206. Extended HELLO (EHLO) or HELLO (HELO) These commands are used to identify the SMTP client to the SMTP server. The argument clause contains the fully-qualified domain name of the SMTP client, if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is
available), the client SHOULD send an address literal (see Section 4.1.3). RFC 2821, and some earlier informal practices, encouraged following the literal by information that would help to identify the client system. That convention was not widely supported, and many SMTP servers considered it an error. In the interest of interoperability, it is probably wise for servers to be prepared for this string to occur, but SMTP clients SHOULD NOT send it. The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command. A client SMTP SHOULD start an SMTP session by issuing the EHLO command. If the SMTP server supports the SMTP service extensions, it will give a successful response, a failure response, or an error response. If the SMTP server, in violation of this specification, does not support any SMTP service extensions, it will generate an error response. Older client SMTP systems MAY, as discussed above, use HELO (as specified in RFC 821) instead of EHLO, and servers MUST support the HELO command and reply properly to it. In any event, a client MUST issue HELO or EHLO before starting a mail transaction. These commands, and a "250 OK" reply to one of them, confirm that both the SMTP client and the SMTP server are in the initial state, that is, there is no transaction in progress and all state tables and buffers are cleared. Syntax: ehlo = "EHLO" SP ( Domain / address-literal ) CRLF helo = "HELO" SP Domain CRLF Normally, the response to EHLO will be a multiline reply. Each line of the response contains a keyword and, optionally, one or more parameters. Following the normal syntax for multiline replies, these keywords follow the code (250) and a hyphen for all but the last line, and the code and a space for the last line. The syntax for a positive response, using the ABNF notation and terminal symbols of RFC 5234 , is: ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) / ( "250-" Domain [ SP ehlo-greet ] CRLF *( "250-" ehlo-line CRLF ) "250" SP ehlo-line CRLF )
ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) ; string of any characters other than CR or LF ehlo-line = ehlo-keyword *( SP ehlo-param ) ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") ; additional syntax of ehlo-params depends on ; ehlo-keyword ehlo-param = 1*(%d33-126) ; any CHAR excluding <SP> and all ; control characters (US-ASCII 0-31 and 127 ; inclusive) Although EHLO keywords may be specified in upper, lower, or mixed case, they MUST always be recognized and processed in a case- insensitive manner. This is simply an extension of practices specified in RFC 821 and Section 2.4. The EHLO response MUST contain keywords (and associated parameters if required) for all commands not listed as "required" in Section 4.5.1 excepting only private-use commands as described in Section 4.1.5. Private-use commands MAY be listed. 220.127.116.11. MAIL (MAIL) This command is used to initiate a mail transaction in which the mail data is delivered to an SMTP server that may, in turn, deliver it to one or more mailboxes or pass it on to another system (possibly using SMTP). The argument clause contains a reverse-path and may contain optional parameters. In general, the MAIL command may be sent only when no mail transaction is in progress, see Section 4.1.4. The reverse-path consists of the sender mailbox. Historically, that mailbox might optionally have been preceded by a list of hosts, but that behavior is now deprecated (see Appendix C). In some types of reporting messages for which a reply is likely to cause a mail loop (for example, mail delivery and non-delivery notifications), the reverse-path may be null (see Section 3.6). This command clears the reverse-path buffer, the forward-path buffer, and the mail data buffer, and it inserts the reverse-path information from its argument clause into the reverse-path buffer. If service extensions were negotiated, the MAIL command may also carry parameters associated with a particular service extension.
Syntax: mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF 18.104.22.168. RECIPIENT (RCPT) This command is used to identify an individual recipient of the mail data; multiple recipients are specified by multiple uses of this command. The argument clause contains a forward-path and may contain optional parameters. The forward-path normally consists of the required destination mailbox. Sending systems SHOULD NOT generate the optional list of hosts known as a source route. Receiving systems MUST recognize source route syntax but SHOULD strip off the source route specification and utilize the domain name associated with the mailbox as if the source route had not been provided. Similarly, relay hosts SHOULD strip or ignore source routes, and names MUST NOT be copied into the reverse-path. When mail reaches its ultimate destination (the forward-path contains only a destination mailbox), the SMTP server inserts it into the destination mailbox in accordance with its host mail conventions. This command appends its forward-path argument to the forward-path buffer; it does not change the reverse-path buffer nor the mail data buffer. For example, mail received at relay host xyz.com with envelope commands MAIL FROM:<firstname.lastname@example.org> RCPT TO:<@hosta.int,@jkl.org:email@example.com> will normally be sent directly on to host d.bar.org with envelope commands MAIL FROM:<firstname.lastname@example.org> RCPT TO:<email@example.com> As provided in Appendix C, xyz.com MAY also choose to relay the message to hosta.int, using the envelope commands MAIL FROM:<firstname.lastname@example.org> RCPT TO:<@hosta.int,@jkl.org:email@example.com>
or to jkl.org, using the envelope commands MAIL FROM:<firstname.lastname@example.org> RCPT TO:<@jkl.org:email@example.com> Attempting to use relaying this way is now strongly discouraged. Since hosts are not required to relay mail at all, xyz.com MAY also reject the message entirely when the RCPT command is received, using a 550 code (since this is a "policy reason"). If service extensions were negotiated, the RCPT command may also carry parameters associated with a particular service extension offered by the server. The client MUST NOT transmit parameters other than those associated with a service extension offered by the server in its EHLO response. Syntax: rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" / Forward-path ) [SP Rcpt-parameters] CRLF Note that, in a departure from the usual rules for local-parts, the "Postmaster" string shown above is treated as case-insensitive. 22.214.171.124. DATA (DATA) The receiver normally sends a 354 response to DATA, and then treats the lines (strings ending in <CRLF> sequences, as described in Section 2.3.7) following the command as mail data from the sender. This command causes the mail data to be appended to the mail data buffer. The mail data may contain any of the 128 ASCII character codes, although experience has indicated that use of control characters other than SP, HT, CR, and LF may cause problems and SHOULD be avoided when possible. The mail data are terminated by a line containing only a period, that is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is actually the terminator of the previous line (see Section 4.5.2). This is the end of mail data indication. The first <CRLF> of this terminating sequence is also the <CRLF> that ends the final line of the data (message text) or, if there was no mail data, ends the DATA command itself (the "no mail data" case does not conform to this specification since it would require that neither the trace header fields required by this specification nor the message header section required by RFC 5322  be transmitted). An extra <CRLF> MUST NOT be added, as that would cause an empty line to be added to the message. The only exception to this rule would arise if the message
body were passed to the originating SMTP-sender with a final "line" that did not end in <CRLF>; in that case, the originating SMTP system MUST either reject the message as invalid or add <CRLF> in order to have the receiving SMTP server recognize the "end of data" condition. The custom of accepting lines ending only in <LF>, as a concession to non-conforming behavior on the part of some UNIX systems, has proven to cause more interoperability problems than it solves, and SMTP server systems MUST NOT do this, even in the name of improved robustness. In particular, the sequence "<LF>.<LF>" (bare line feeds, without carriage returns) MUST NOT be treated as equivalent to <CRLF>.<CRLF> as the end of mail data indication. Receipt of the end of mail data indication requires the server to process the stored mail transaction information. This processing consumes the information in the reverse-path buffer, the forward-path buffer, and the mail data buffer, and on the completion of this command these buffers are cleared. If the processing is successful, the receiver MUST send an OK reply. If the processing fails, the receiver MUST send a failure reply. The SMTP model does not allow for partial failures at this point: either the message is accepted by the server for delivery and a positive response is returned or it is not accepted and a failure reply is returned. In sending a positive "250 OK" completion reply to the end of data indication, the receiver takes full responsibility for the message (see Section 6.1). Errors that are diagnosed subsequently MUST be reported in a mail message, as discussed in Section 4.4. When the SMTP server accepts a message either for relaying or for final delivery, it inserts a trace record (also referred to interchangeably as a "time stamp line" or "Received" line) at the top of the mail data. This trace record indicates the identity of the host that sent the message, the identity of the host that received the message (and is inserting this time stamp), and the date and time the message was received. Relayed messages will have multiple time stamp lines. Details for formation of these lines, including their syntax, is specified in Section 4.4. Additional discussion about the operation of the DATA command appears in Section 3.3. Syntax: data = "DATA" CRLF
126.96.36.199. RESET (RSET) This command specifies that the current mail transaction will be aborted. Any stored sender, recipients, and mail data MUST be discarded, and all buffers and state tables cleared. The receiver MUST send a "250 OK" reply to a RSET command with no arguments. A reset command may be issued by the client at any time. It is effectively equivalent to a NOOP (i.e., it has no effect) if issued immediately after EHLO, before EHLO is issued in the session, after an end of data indicator has been sent and acknowledged, or immediately before a QUIT. An SMTP server MUST NOT close the connection as the result of receiving a RSET; that action is reserved for QUIT (see Section 188.8.131.52). Since EHLO implies some additional processing and response by the server, RSET will normally be more efficient than reissuing that command, even though the formal semantics are the same. There are circumstances, contrary to the intent of this specification, in which an SMTP server may receive an indication that the underlying TCP connection has been closed or reset. To preserve the robustness of the mail system, SMTP servers SHOULD be prepared for this condition and SHOULD treat it as if a QUIT had been received before the connection disappeared. Syntax: rset = "RSET" CRLF 184.108.40.206. VERIFY (VRFY) This command asks the receiver to confirm that the argument identifies a user or mailbox. If it is a user name, information is returned as specified in Section 3.5. This command has no effect on the reverse-path buffer, the forward- path buffer, or the mail data buffer. Syntax: vrfy = "VRFY" SP String CRLF
220.127.116.11. EXPAND (EXPN) This command asks the receiver to confirm that the argument identifies a mailing list, and if so, to return the membership of that list. If the command is successful, a reply is returned containing information as described in Section 3.5. This reply will have multiple lines except in the trivial case of a one-member list. This command has no effect on the reverse-path buffer, the forward- path buffer, or the mail data buffer, and it may be issued at any time. Syntax: expn = "EXPN" SP String CRLF 18.104.22.168. HELP (HELP) This command causes the server to send helpful information to the client. The command MAY take an argument (e.g., any command name) and return more specific information as a response. This command has no effect on the reverse-path buffer, the forward- path buffer, or the mail data buffer, and it may be issued at any time. SMTP servers SHOULD support HELP without arguments and MAY support it with arguments. Syntax: help = "HELP" [ SP String ] CRLF
22.214.171.124. NOOP (NOOP) This command does not affect any parameters or previously entered commands. It specifies no action other than that the receiver send a "250 OK" reply. This command has no effect on the reverse-path buffer, the forward- path buffer, or the mail data buffer, and it may be issued at any time. If a parameter string is specified, servers SHOULD ignore it. Syntax: noop = "NOOP" [ SP String ] CRLF 126.96.36.199. QUIT (QUIT) This command specifies that the receiver MUST send a "221 OK" reply, and then close the transmission channel. The receiver MUST NOT intentionally close the transmission channel until it receives and replies to a QUIT command (even if there was an error). The sender MUST NOT intentionally close the transmission channel until it sends a QUIT command, and it SHOULD wait until it receives the reply (even if there was an error response to a previous command). If the connection is closed prematurely due to violations of the above or system or network failure, the server MUST cancel any pending transaction, but not undo any previously completed transaction, and generally MUST act as if the command or transaction in progress had received a temporary error (i.e., a 4yz response). The QUIT command may be issued at any time. Any current uncompleted mail transaction will be aborted. Syntax: quit = "QUIT" CRLF 188.8.131.52. Mail-Parameter and Rcpt-Parameter Error Responses If the server SMTP does not recognize or cannot implement one or more of the parameters associated with a particular MAIL FROM or RCPT TO command, it will return code 555. If, for some reason, the server is temporarily unable to accommodate one or more of the parameters associated with a MAIL FROM or RCPT TO command, and if the definition of the specific parameter does not mandate the use of another code, it should return code 455.
Errors specific to particular parameters and their values will be specified in the parameter's defining RFC. 4.1.2. Command Argument Syntax The syntax of the argument clauses of the above commands (using the syntax specified in RFC 5234  where applicable) is given below. Some of the productions given below are used only in conjunction with source routes as described in Appendix C. Terminals not defined in this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in the "core" syntax in Section 6 of RFC 5234  or in the message format syntax in RFC 5322 . Reverse-path = Path / "<>" Forward-path = Path Path = "<" [ A-d-l ":" ] Mailbox ">" A-d-l = At-domain *( "," At-domain ) ; Note that this form, the so-called "source ; route", MUST BE accepted, SHOULD NOT be ; generated, and SHOULD be ignored. At-domain = "@" Domain Mail-parameters = esmtp-param *(SP esmtp-param) Rcpt-parameters = esmtp-param *(SP esmtp-param) esmtp-param = esmtp-keyword ["=" esmtp-value] esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") esmtp-value = 1*(%d33-60 / %d62-126) ; any CHAR excluding "=", SP, and control ; characters. If this string is an email address, ; i.e., a Mailbox, then the "xtext" syntax  ; SHOULD be used. Keyword = Ldh-str Argument = Atom Domain = sub-domain *("." sub-domain)
sub-domain = Let-dig [Ldh-str] Let-dig = ALPHA / DIGIT Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig address-literal = "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]" ; See Section 4.1.3 Mailbox = Local-part "@" ( Domain / address-literal ) Local-part = Dot-string / Quoted-string ; MAY be case-sensitive Dot-string = Atom *("." Atom) Atom = 1*atext Quoted-string = DQUOTE *QcontentSMTP DQUOTE QcontentSMTP = qtextSMTP / quoted-pairSMTP quoted-pairSMTP = %d92 %d32-126 ; i.e., backslash followed by any ASCII ; graphic (including itself) or SPace qtextSMTP = %d32-33 / %d35-91 / %d93-126 ; i.e., within a quoted string, any ; ASCII graphic or space is permitted ; without blackslash-quoting except ; double-quote and the backslash itself. String = Atom / Quoted-string While the above definition for Local-part is relatively permissive, for maximum interoperability, a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form or where the Local-part is case- sensitive. For any purposes that require generating or comparing Local-parts (e.g., to specific mailbox names), all quoted forms MUST be treated as equivalent, and the sending system SHOULD transmit the form that uses the minimum quoting possible. Systems MUST NOT define mailboxes in such a way as to require the use in SMTP of non-ASCII characters (octets with the high order bit set
to one) or ASCII "control characters" (decimal value 0-31 and 127). These characters MUST NOT be used in MAIL or RCPT commands or other commands that require mailbox names. Note that the backslash, "\", is a quote character, which is used to indicate that the next character is to be used literally (instead of its normal interpretation). For example, "Joe\,Smith" indicates a single nine-character user name string with the comma being the fourth character of that string. To promote interoperability and consistent with long-standing guidance about conservative use of the DNS in naming and applications (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 ), characters outside the set of alphabetic characters, digits, and hyphen MUST NOT appear in domain name labels for SMTP clients or servers. In particular, the underscore character is not permitted. SMTP servers that receive a command in which invalid character codes have been employed, and for which there are no other reasons for rejection, MUST reject that command with a 501 response (this rule, like others, could be overridden by appropriate SMTP extensions). 4.1.3. Address Literals Sometimes a host is not known to the domain name system and communication (and, in particular, communication to report and repair the error) is blocked. To bypass this barrier, a special literal form of the address is allowed as an alternative to a domain name. For IPv4 addresses, this form uses four small decimal integers separated by dots and enclosed by brackets such as [184.108.40.206], which indicates an (IPv4) Internet Address in sequence-of-octets form. For IPv6 and other forms of addressing that might eventually be standardized, the form consists of a standardized "tag" that identifies the address syntax, a colon, and the address itself, in a format specified as part of the relevant standards (i.e., RFC 4291  for IPv6). Specifically: IPv4-address-literal = Snum 3("." Snum) IPv6-address-literal = "IPv6:" IPv6-addr General-address-literal = Standardized-tag ":" 1*dcontent Standardized-tag = Ldh-str ; Standardized-tag MUST be specified in a ; Standards-Track RFC and registered with IANA
dcontent = %d33-90 / ; Printable US-ASCII %d94-126 ; excl. "[", "\", "]" Snum = 1*3DIGIT ; representing a decimal integer ; value in the range 0 through 255 IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp IPv6-hex = 1*4HEXDIG IPv6-full = IPv6-hex 7(":" IPv6-hex) IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":" IPv6-hex)] ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 6 groups in addition to the ; "::" may be present. IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::" [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal ; The "::" represents at least 2 16-bit groups of ; zeros. No more than 4 groups in addition to the ; "::" and IPv4-address-literal may be present. 4.1.4. Order of Commands There are restrictions on the order in which these commands may be used. A session that will contain mail transactions MUST first be initialized by the use of the EHLO command. An SMTP server SHOULD accept commands for non-mail transactions (e.g., VRFY or EXPN) without this initialization. An EHLO command MAY be issued by a client later in the session. If it is issued after the session begins and the EHLO command is acceptable to the SMTP server, the SMTP server MUST clear all buffers and reset the state exactly as if a RSET command had been issued. In other words, the sequence of RSET followed immediately by EHLO is redundant, but not harmful other than in the performance cost of executing unnecessary commands. If the EHLO command is not acceptable to the SMTP server, 501, 500, 502, or 550 failure replies MUST be returned as appropriate. The
SMTP server MUST stay in the same state after transmitting these replies that it was in before the EHLO was received. The SMTP client MUST, if possible, ensure that the domain parameter to the EHLO command is a primary host name as specified for this command in Section 2.3.5. If this is not possible (e.g., when the client's address is dynamically assigned and the client does not have an obvious name), an address literal SHOULD be substituted for the domain name. An SMTP server MAY verify that the domain name argument in the EHLO command actually corresponds to the IP address of the client. However, if the verification fails, the server MUST NOT refuse to accept a message on that basis. Information captured in the verification attempt is for logging and tracing purposes. Note that this prohibition applies to the matching of the parameter to its IP address only; see Section 7.9 for a more extensive discussion of rejecting incoming connections or mail messages. The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time during a session, or without previously initializing a session. SMTP servers SHOULD process these normally (that is, not return a 503 code) even if no EHLO command has yet been received; clients SHOULD open a session with EHLO before sending these commands. If these rules are followed, the example in RFC 821 that shows "550 access denied to you" in response to an EXPN command is incorrect unless an EHLO command precedes the EXPN or the denial of access is based on the client's IP address or other authentication or authorization-determining mechanisms. The MAIL command (or the obsolete SEND, SOML, or SAML commands) begins a mail transaction. Once started, a mail transaction consists of a transaction beginning command, one or more RCPT commands, and a DATA command, in that order. A mail transaction may be aborted by the RSET, a new EHLO, or the QUIT command. There may be zero or more transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be sent if a mail transaction is already open, i.e., it should be sent only if no mail transaction had been started in the session, or if the previous one successfully concluded with a successful DATA command, or if the previous one was aborted, e.g., with a RSET or new EHLO. If the transaction beginning command argument is not acceptable, a 501 failure reply MUST be returned and the SMTP server MUST stay in the same state. If the commands in a transaction are out of order to the degree that they cannot be processed by the server, a 503 failure
reply MUST be returned and the SMTP server MUST stay in the same state. The last command in a session MUST be the QUIT command. The QUIT command SHOULD be used by the client SMTP to request connection closure, even when no session opening command was sent and accepted. 4.1.5. Private-Use Commands As specified in Section 2.2.2, commands starting in "X" may be used by bilateral agreement between the client (sending) and server (receiving) SMTP agents. An SMTP server that does not recognize such a command is expected to reply with "500 Command not recognized". An extended SMTP server MAY list the feature names associated with these private commands in the response to the EHLO command. Commands sent or accepted by SMTP systems that do not start with "X" MUST conform to the requirements of Section 2.2.2.