Re: [exim] Implementing a secondary MX

On Tue, 2005-01-04 at 22:49 +0000, David Cannings wrote:
> For example, the primary MX receives a mail from the backup MX and
> decides it doesn't like it (spam, stricter ACLs, no-such-mailbox etc.)
> so rejects it. The backup is then going to attempt to send out a bounce
> to the originating host. The no-such-mailbox problem could be countered
> by having a list of valid users on the backup MX, that isn't hard to
> solve. However, I see no way to fix the first two.

Just make sure you have the same (or stricter) spam and other ACL checks
on the backup.

Remember that if in doubt you can _defer_ suspicious mail from the MX
backup, and hence force the sender to wait until the primary is up. You
only have to accept mail on the backup that you're _sure_ is good.

You don't have to duplicate the user list either -- you can do recipient
verification SMTP callouts. In the majority of cases the primary will
actually be alive, and you'll get to reject mail to unknown users. Only
in the rare case that the primary is actually unreachable might you
accept mail which you later have to bounce.

--
dwmw2