decided to accept the message, it must assume the message has been
delivered to it. If the communications link fails during this
synchronization gap, then the message has been duplicated. Both
mailers have active copies of the message that they will try to
It may be hard to believe that this problem is the cause of many
duplicate messages. Intuitively, one might expect that the time
spent in the state between the final dot and its accepting 250 reply
is quite small. In practice, however, this period is often quite
long; long enough that timeouts by the sending mailer (or possibly
network failures) are quite common. Observations by the author
suggest that this synchronization problem may be the second leading
cause of duplicate messages on the Internet (second to mail loops).
Many mailers delay responding to the final dot because they are doing
sophisticated processing of the message, in an attempt to confirm
that they can deliver the message. For example, the mailers may
expand an entire mailing list to confirm that it can reach all
addressees or may attempt to physically deposit the message into the
mailboxes of local users, before confirming receipt of the final dot.
These practices are not unreasonable, but they often cause the
synchronization gap to continue for several minutes, and increase the
likelihood that the sending mailer will timeout or the network will
fail before the accepting 250 reply is sent.
AVOIDING SYNCHRONIZATION PROBLEMS
The best way to avoid the synchronization problem is to minimize the
length of the synchronization gap. In other words, receiving mailers
should acknowledge the final dot as soon as possible and do more
complex processing of the message later.
RFC-821 (on page 22) states that unless the receiving mailer is
completely unable to process a message it should accept the message
and acknowledge any errors in processing in a separate message or
messages sent back to the originator of the message. As a result,
receiving mailers should be able to acknowledge the final dot as soon
as the message has been safely put in a non-volatile (e.g., disk)
queue for further processing. Fast acceptance of a message does not
Some mailers can be configured to do more or less processing upon
receipt of the final dot. In such situations, the mailer should
always be configured to do less processing.
Finally, some mailers allow remote mailers only a minute or two to
acknowledge the final dot before timing out and trying again. Given