[Mimedefang] Off Topic: Refuse Null Sender

[Joseph Brennan]

--On Tuesday, July 3, 2007 12:30 -0400 Jason Bertoch <jason at electronet.net> 
wrote:

> Hi all, sorry to post off topic but searches for this only seem to return
> reasons not to attempt.  However, I have a server that is exclusively
> used to relay outbound mail.  I can't think of any legitimate reason for
> someone to relay mail from the null sender.  Blocking this will seem to
> eliminate some back-scatter as well as botnet spam.  I've tried various
> methods with the access table to no avail.  Any ideas?


You'd have to know for certain that none of the mail you relay would
ever be any kind of delivery status notice.  Even if none of the hosts
runs an MTA (can you be sure?) note that some clients send read receipts
and autoreplies which would properly be from <>.

It is easily done in mimedefang...

if ($Sender eq '<>') {
  action_bounce('No mail from null sender');
}

...although now you have to be sure the host getting the 550 will not
send a notice from <> to your user to report this failure!

Is this actually a problem that needs to be solved?

Joseph Brennan
Lead Email Systems Engineer
Columbia University Information Technology