[Gllug] Rejecting mail at backup MX

[Jason Clifford]

On Tue, 10 Feb 2004, Bruce Richardson wrote:

> Right.  Well, if your backup MX were running Exim, you could use its
> "call forward" feature where it can (if so configured) check the
> recipients on incoming mail with the end system that it will eventually
> deliver the mail to and reject the recipient if the end system does so.
> It does this for each recipient as they are given in the course of the
> smtp conversation, so no NDRs are generated by your system.  If the end
> system can't be contacted then it can optionally defer or accept the
> recipient.

My experience is that there are only two types of connection made to a 
backup MX while the primary is available:

1. the vast majority are spam.
2. The others come from fundementally broken implementations of SMTP that 
fail to obey MX priority values and there are very few of these.

> I haven't used Postfix for anything serious in some time (come to think
> of it, I replaced the last Postfix set-up a few weeks ago), so I don't
> know if it has a similar feature.  I'd be interested to know if it does.
> If not, your practical alternatives are LDAP lookups (which you said you
> don't want) or synchronising user maps between the two boxes.

Synchronised maps can be very easily done, particularly when using sql 
based maps with replication.  This provides the secondary benefit of 
having backups of your maps on an easy to access location.

Jason Clifford
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