On Sat, 2011-02-26 at 12:08 +0000, Chris Wilson wrote:
> Option 3) have the secondary only accept mail when the primary is down
> (possibly combined with 2):
> http://blog.aptivate.org/2009/01/28/backup-mail-exchangers/
While that's perfectly valid, there are any number of reasons why a
given source address would be able to connect to your secondary MX but
not the primary. Almost none of those reasons are under your control,
nor that of the source. If you're running a high-volume system (in terms
of rate, or end users) then this will turn round and bite at some point.
As Jeremy mentioned, good practice is to ensure both primary and
secondary (tertiary, etc) MX hosts for a given domain have the same
"knowledge" of the domain's recipients. There are hundreds of ways to
achieve that, aside from doing call-forward verification.
Graeme
