7/24 Sendmail/DNS question: If A has a MX 0 to B and B is MX 0 to itself
and MX 10 to C, does A need an MX 10 to C or will mail to foo@A
automagically go to C if B is down?
\_ MXes don't chain; the target of an MX is a hostname, not another
target for an MX lookup:
; domain file for http://bar.com
mail IN MX 10 http://mail.foo.com.
; SMTP requires doing an MX lookup for http://bar.com, which will give it
; back http://mail.foo.com. it will _not_ do an MX lookup on http://mail.foo.com,
; merely an A record lookup (ordinary "name lookup")
\- if there is not second MX for A -> C, then the mail will wait
for B to come back. If there is a secondary MX for A -> C and
B is down, I *believe* the answer is "it depends" ... you arent
obligated to try another MX I think. Dont quote me on this.
Recent sendmail should make one pass through them until one
works or all of them are down, unless you get some exotic SMTP
code which halts the whole process. If you are 100% sendmail see
BAT II. If you have mixed agents, good luck reading the RFCs and
sorting out the "musts" and "shoulds". This is subject to your
not doing any of the weird things that turn off MX lookups. --psb |