[Maybe OT] - RFC compliance checking at session
Hostmaster at computerservicecentre.com
Fri Feb 29 15:05:30 GMT 2008
I would like to illicit some opinions from you other MailScanner using
MX-administrators. I know that there was some discussion on list some time ago
regarding session checking, particularly HELO/EHLO checking, and its compliance
against RFC 821, as clarified and updated in 2821.
We use Exim for both inbound and outbound message handling around MailScanner,
and on the inbound, some quite complex ACL's to validate the session to try and
cut down the amount of spam our users get. The first check we run is to ensure
that the HELO/EHLO is an FQDN. We don't then validate if this FQDN can be
resolved, or even if it is valid, it just has to be host.domain.tld, and this
significantly cuts the number of RBL lookups we do. This hasn't caused us any
problems with rejecting valid mail until now.
One of our users complained that they were no longer receiving a newsletter they
signed up for. I managed to find it in the exim reject logs, and sure enough, it
was failing the host checking - the EHLO it sends is "(server3549)", and exim
declines the session with a 550 - permanent reject for policy reasons.
Now comes the fun part. That 550 is not enough for the sender - it ignores it
and constantly retries the send, treating it more like a 450, but not following
any normal MTA retry period I can establish. That would be enough for me to
leave them blocked, but checking further, the IP for that host has no RDNS, also
a big no-no in my opinion for a valid mail server, and the IP does not accept
return SMTP - indicating that it's probably a web server and not an MTA itself.
I even took the liberty of doing an IPWhois, phoning the helpdesk of the company
responsible for the IP (only because they are UK based the same as us) and
pointing the problem out, only to be met with "yeah, we know about that, it'll
be fixed sometime next year when we put a new server in", even after I pointed
out that they wouldn't be getting successful deliveries to organisations such as
AOL (RDNS is a must) and BT/Yahoo (whose policies are incredibly strict)!
So what do you guys think? Am I just being particularly awkward on a Friday
afternoon and should I spend my time re-working our config to work around an
organisation who is blatantly ignorant of common mail server practise, or just
tell my user that the sending organisation needs to get their act together?
Richard Garner (A+, N+, AMBCS, MOS-O)
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