[Mimedefang] Strange duplicate mail

[Chris Myers]

----- Original Message ----- 
From: "James Miller" <jimm at simutronics.com>
To: <mimedefang at lists.roaringpenguin.com>
Sent: Wednesday, October 15, 2003 5:07 PM
Subject: [Mimedefang] Strange duplicate mail


> I'm running MimeDefang 2.35 and SA 2.55 w/Sendmail 8.12.9.  Occasionally
> (more an more frequently) we are getting duplicate messages.  The
duplicate
> messages have the same message ID from sender; we assign a unique id to
each
> of the duplicates.
>
> [...]
>
> After the '<enter> . <enter>' it's taking 60-70 seconds to accept the
> message.  I think some of the servers sending us mail are closing the
> connection before we acknowledge receipt -- and later resends the message
on
> it next queue run -- though we did, in fact, get the message.


Some mail servers use a very short timeout for the response to the DATA
command, as short as 30 seconds in some cases (despite the fact that the RFC
states that it should be a minimum of 10 MINUTES).  What happens is that the
sending MTA gives up before your system's response to the DATA command, but
your system doesn't detect that and goes on to deliver the message.  The
sending MTA thinks there was a failure and re-queues the message.

This absolutely will cause the sending MTA to repeatedly deliver the same
message to your system.  It's absolutely the fault of the sender's system,
but your chances of getting them to fix it are likely low.

It's actually sendmail on your system that's responsible for detecting that
the sender has gone away, and it really doesn't have a way to do that except
after it has already sent the "successful" response to the DATA command.
This isn't MIMEDefang's responsibility.

The more appropriate place to look into is why your system is taking up to
70 seconds to process a message and work to get it down below 30 seconds.
The likely cause is SpamAssassin -- you mentioned that you are using SA
2.55, which uses several DNS blocking lists that are defunct ... the DNS
lookups take quite a while to timeout.

Chris Myers
Networks By Design