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

On 11/14/11 7:35 AM, "Marc Perkel" <marc@???> wrote:

>The way I determine if the primary is down is to do a forward callout.
>If it returns a message that it can't connect or complete verification
>then the server is offline and the backup should take the message. if it
>is verified as good then the primary is online and should return a defer
>- 4xx error.

Some broken MTAs-- I'm thinking Qmail, but I may be mistaken-- will "lock
on" to an MX that responds with a 4xx error and will only retry sending to
that server instead of going through the normal progression of MX records
again. In that case, a temporary outage on the primary could lead to a
message never being delivered.

Also, be aware that sometimes routing issues, firewall problems, etc. may
cause the primary to be unreachable from one site but reachable from the
secondary; again, this could cause delivery problems.

Just a couple of things to consider.
--
Dave Pooser
Cat-Herder-in-Chief, Pooserville.com
"If you think bringing a wet noodle to an Amish rake fight
makes you 'better armed' for it, then you have a funny
understanding of how the universe works." -- Al Iverson