[Mimedefang] SMTP Pipelining, and GREYLISTING

[David F. Skoll]

On Fri, 26 Mar 2004, Steffen Kaiser wrote:

> And why is pipelining a problem with greylisting only?

Pipelining and greylisting interact badly with certain broken mail servers.
Let's say you're greylisting and you tempfail all RCPT commands.  Sendmail
replies with a 4xx failure code for all your RCPTs, and then it fails the
DATA command with a 5xx failure code!  It does that because you are *not*
supposed to issue DATA unless at least one RCPT has succeeded.

Decent MTA's will understand why the DATA got a 5xx failure code, and will
queue the mail for retry.  Buggy ones will see the 5xx failure code
and bounce the message.

For our commercial software, we have an option to defer greylist processing
until after the DATA phase (ie, while the client is waiting for the
response after the ".").  This wastes bandwidth and isn't as "elegant"
as doing it at RCPT time, but it avoids any problems with pipelining
and buggy MTA's.

Regards,

David.