Re: [Exim] pri/sec MX on same machine to catch spam

Wakko Warner wrote:

> Am I correct in assuming that most mailservers (exim, sendmail, qmail,
> smail, postfix, ...) will try the backup MX if the primary is down or
> returns a temporary failure code? If so and having 2 MXs on seperate
> machines, I'm not sure how to ensure that the 2nd has the information before
> the mailserver gives up on #1 and tries #2.
>

If primary is down (doesn't answer TCP open), yes, secondary will be
tried. If primary returns 4xx response code, no, secondary should not
be tried. Not to say that some systems out there don't do it, but as
far as I'm aware if the MTA was able to open a TCP connection to the
lowest-priority MX and get at least an opening banner (without a
permanent error response code), it shouldn't try higher-priority MXs.