Re: 5.1.1 User unknown bounces

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On Sat, 9 Sep 2006, Dave McCracken wrote:

On Saturday 09 September 2006 9:19 am, Kjetil Torgrim Homme wrote:
On Fri, 2006-09-08 at 10:45 -0700, Andrew Morgan wrote:
Otherwise, just let Sendmail queue the message and attempt to deliver the
message to Cyrus.  If the user does not exist, Cyrus will let Sendmail
know during the LMTP handshake.

DO NOT DO THIS!  if your Sendmail accepts _all_ possible local parts
during the SMTP transaction, you will be sending out lots of bogus
bounces to addresses abused (joe-jobbed) as senders of spam.

I second this emphatically.  I discovered I was sending out thousands of
bounce messages per day with this setup.

My solution was to go into my sendmail.mc and define "CYRUSV2_MAILER_FLAGS"
to be "A@/:|mw".  The default does not have the "w" flag.  This flag tells
sendmail to validate the user id on the local machine when it queues the mail
for this mailer.  Since I have a small set of valid users it was easy for me
to define them all in /etc/passwd.  I'd guess a larger site would want to set
up something more complex.

The key point is that sendmail still has the connection to the sender open
when it selects the mailer.  If it detects an error there it responds with an
error status to the sending mailer.  If no error is detected, sendmail will
close the connection before actually invoking the mailer.  At this point its
only recourse is to send bounce mail.

To my knowledge, Postfix does not support the socket map protocol for verifying a mailbox exists during the SMTP transaction. I guess the Postfix users are just screwed on this then. :)

In our case, our campus mail relays (6 of them currently) accept mail for all domains on campus and perform RBL and spam tagging before relaying the messages to their final destinations. You'll have to live with the bounce messages coming from our domain. :P

	Andy
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