5.1.1 User unknown bounces

[Dave McCracken]

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.

Dave McCracken