The OBJECTID capability of IMAP (RFC 8474) allows clients to identify mailboxes by a unique identifier that survives renaming.
This document extends the Sieve email filtering language (RFC 5228) to allow using that same unique identifier as a target for fileinto rules and for testing the existence of mailboxes.
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Sieve rules [RFC 5228] are sometimes created using graphical interfaces, which allow users to select the mailbox to be used as a target for a rule.
If that mailbox is renamed, the client may also update its internal representation of the rule and update the Sieve script to match; however, this is a multistep process and subject to partial failures. Also, if the folder is renamed by a different mechanism (e.g., another IMAP client), the rules will get out of sync.
By telling fileinto to reference the immutable MAILBOXID specified by [RFC 8474], using the extension specified herein, Sieve rules can continue to target the same mailbox, even if it gets renamed.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.
Normally, the fileinto command delivers the message in the mailbox specified using its positional mailbox argument. However, if the optional :mailboxid argument is also specified, the fileinto command first checks whether a mailbox exists in the user's personal namespace [RFC 2342] with the specified MAILBOXID [RFC 8474].
If a matching mailbox is found, that mailbox is used for delivery.
If there is no such mailbox, the fileinto action proceeds as it would without the :mailboxid argument.
The tagged argument :mailboxid to fileinto consumes one additional token, a string containing the OBJECTID of the target mailbox.
For servers that also support the mailbox extension defined in [RFC 5490], if both the :create and :mailboxid arguments are provided to a fileinto command and no matching mailbox is found, then a new mailbox will be created.
This new mailbox will have the name specified by the positional mailbox argument (RFC 5228, Section 4.1); however, it will get a different MAILBOXID (chosen by the server) rather than the one specified by the :mailboxid argument to fileinto.
fileinto :mailboxid "Fnosuch"
# creates INBOX.no-such-folder, but it doesn't
# get the "Fnosuch" mailboxid.
For servers that also support delivery to special-use mailboxes [RFC 8579], it is an error to specify both :mailboxid and :specialuse in the same fileinto command.
Advanced filtering based on both special-use and MAILBOXID can be built with explicit specialuse_exists and mailboxidexists tests.
This document extends the definition of the :fcc argument defined in [RFC 8580] so that it can optionally be used with the :mailboxid argument. The syntax for FCC is extended here using ABNF [RFC 5234]:
If the optional :mailboxid argument is specified with :fcc, it instructs the Sieve interpreter to check whether a mailbox exists with the specific MAILBOXID. If such a mailbox exists, the generated message is filed into that mailbox. Otherwise, the generated message is filed into the :fcc target mailbox.
As with fileinto, it is an error to specify both :mailboxid and :specialuse for the same fcc rule.
Usage: mailboxidexists <mailbox-objectids: string-list>
The mailboxidexists test is true if every string argument provided is the MAILBOXID of a mailbox that exists in the mailstore and that allows the user in whose context the Sieve script runs to deliver messages into it.
When the mailstore is an IMAP server that also supports IMAP Access Control List (ACL) [RFC 4314], delivery is allowed if the user has the 'p' or 'i' rights for the mailbox (see Section 5.2 of RFC 4314).
When the mailstore is an IMAP server that does not support IMAP ACL, delivery is allowed if the READ-WRITE response code is present for the mailbox when selected by the user (see Section 7.1 of RFC 3501).
Note that a successful mailboxidexists test for a mailbox doesn't necessarily mean that a "fileinto :mailboxid" action on this mailbox would succeed. For example, the fileinto action might put the user over quota. The mailboxidexists test only verifies existence of the mailbox and whether the user in whose context the Sieve script runs has permissions to execute fileinto on it.
Because MAILBOXID is always generated by the server, implementations MUST NOT allow Sieve to make an end run around this protection by creating mailboxes with the specified ID by using :create and :mailboxid in a fileinto rule for a nonexistent mailbox.
Implementers are referred to the Security Considerations sections of [RFC 5228] and [RFC 8474].
A. Melnikov, "The Sieve Mail-Filtering Language -- Extensions for Checking Mailbox Status and Accessing Mailbox Metadata", RFC 5490, DOI 10.17487/RFC5490, March 2009, <https://www.rfc-editor.org/info/rfc5490>.
This document borrows heavily from [RFC 5490] for the matching mailboxexists test and from [RFC 8579] for an example of modifying the fileinto command.
Thanks to Ned Freed, Ken Murchison, and Alexey Melnikov for feedback on the EXTRA mailing list.