On 11/14/2011 5:47 AM, Graeme Fowler wrote:
On Mon, 2011-11-14 at 05:35 -0800, Marc Perkel wrote:
I'm still playing with this. Probable also need to reduce or eliminate
caching on the recipient callout. But I thought I'd post it in case
someone finds it useful.
It's a fairly old technique but it's hidebound by problems - probably
more than it solves.
You can never assume that an inbound SMTP connection to a lower priority
MX server is coming in when the higher priority server(s) is (are) up
means that the inbound message is spam. It may be so that there's a
higher probability that it is, but consider the fact that there are an
infinity of reasons why a given host can't talk to your higher prio MX.
Network interruptions are the principal cause, but also the higher prio
could have deferred or refused the connection due to a volume of inbound
messages. That doesn't mean it's down, but is in self-defence mode - and
in many cases, will still be accepting from the lower prio MX servers
*especially* if you're doing call-forward to them!
It can be a useful scoring method, but you cannot absolutely assert that
the inbound message is spam.
Be prepared for false positives if you use this technique to make that
assertion.
Graeme
I'm not assuming it's spam. That's wy I'm returning a 4xx response. The
idea is that if it is legit it will be forced back to the primary. This
way the backup MX servers are only active when the primary is actually down.
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