postfix-users October 2010 archive
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postfix-users: SMTP relay and greylisting

SMTP relay and greylisting

From: utahnix <utahnix_at_nospam>
Date: Tue Oct 26 2010 - 02:38:55 GMT
To: postfix <postfix-users@postfix.org>

Hello all,

This question has probably been asked on this list before, but maybe not
quite with these circumstances. I'm hoping one of you can give me some
direction.

I've got a fairly typical Postfix setup... Postfix, Cyrus IMAP, ClamAV,
SpamAssassin... all on Linux.

Anyway, I've set up greylisting with Postgrey to help cut down on the
junk mail that I get. I've set it up with default values (deferral of
300 seconds, etc). Well all seems good and fair except some of my
regular senders can't seem to get their email through. I've checked my
server logs and I don't even see their email address mentioned (it
doesn't appear to even reach my machine). Several of the emails in
question are Yahoo or Gmail. What's odd is that I have both a Yahoo
account and a Gmail account, and I can send myself mail with no problems.

I disabled Postgrey temporarily and had these senders re-send test
messages from their addresses, and it worked (I got their messages). So
something was certainly hanging things up. I just wish I knew what that was.

This got me thinking... my ISP requires that I forward all outbound
email through their SMTP server. Because their mail server (the SMTP
relay I'm required to relay mail to) has suddenly been added to various
RBLs for repeated "deferrals", is it possible that my greylisting is
what is getting them on those RBLs?

The Postgrey does cut down on the spam significantly, particularly when
used in conjunction with SpamAssassin and RBLs like SpamCop and
SpamHaus. I'd like to keep Postgrey if I can, assuming that my delivery
problems are not directly associated with Postgrey... but if my
circumstances with my ISP won't allow me to greylist, then disabling
Postgrey might save me a headache.

I guess I'm looking for some advice as to whether Postgrey could cause
problems with my ISP (they run Exim on FreeBSD and firewall outgoing tcp
port 25 everywhere but on their one mail server) but I don't know much
more than that), or if there are some settings I should change to
improve my greylisting setup.

And just to clarify, port 25 is only blocked on the outbound. Inbound
email comes straight to my mail system, which is only composed of one
machine.

Advice is very appreciated!