Re: [exim] Spam Blocking Trick on backup MX servers

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.

>