MX priority mechanism question

Frank Joerdens frank at joerdens.de
Wed Apr 28 15:33:53 UTC 2004


Greetings! What I am wondering about is this: If the first MX is not
reachable, and the SMTP daemon then queries the nameserver for the next
exchanger (or does the SMTP daemon query both to begin with and stores
this information?), and that turns out to be unreachable too, will the
SMTP daemon then try the first again, or give up eventually?

My guess would be that it depends on the SMTP software you're using (But
where would this be documented for e.g. Sendmail or Postfix?).

The reason I am asking is that I just put in a 2ndary MX for all of the
domains (about a dozen) on our mail server because the router on the
leased line that is used for our mail traffic had to be replaced.  My
thinking being: If we run into the unexpected, and the line is down for
longer than a few minutes, I'll change the default route on the mail
server to our other gateway (which corresponds to a differnet DNS name,
a dyndns.org name actually) - which is where the MX record with the
lower priority points now.

Then I just made a potentially (can't tell because I don't understand
the mechanism) rather evil discovery: I tried to send mail from a host
on our network to one of the domains on our mail server, and checked the
mail log, which kept saying that the 2ndary exchanger was not reachable.
I don't know how this host (or SMTP daemon) came to try the 2ndary in
the first place, because the primary was already back on line by that
time. However, I had to start Postfix to make it try the primary again
... all of which makes me think now that my plan might have been a VERY
BAD IDEA if it means that every time an SMTP daemon out there doesn't
get the primary straight away, it'll try the 2ndary - which won't work
since the default route points to the other gateway - and then give up
eventually.

Any ideas?

Regards,

Frank


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