SUMMARY: name resolution problems on mail server

Thanks to this list, we have both a resolution and solution to the problem!

Thanks go out to:
Ken McKinlay,
Chris Pinnock
Luc Suryo
Adrian Cole
Darren Dunham
Joon Hansen
Jerry K.
Wolfgang Leideck
Hutin Bertrand

The problem turned out to be our primary DNS failing, and sending corrupt
zone data to our secondary. Our sendmail servers shouldn't actually use DNS
for looking up our mailhub, but sendmail will always look up an MX record
for a destination, before reverting to host lookup. What we saw was that
the sendmail servers were looking up an MX record against our secondary for
"exchange.ourdomain.com", failing, and then stopping. Once the primary was
fixed, the secondary got its new zone info, and everything worked properly
again against either DNS server. Some interesting lessons fell out of this,
though.

- Many people reminded me to check /etc/nsswitch.conf to make sure that
  files was ahead of DNS (it was).
- A few suggested setting the smarthost/forwarding host to an IP address,
  in order to avoid DNS lookups. This should work, but it must be escaped,
  just like a hostname. Namely:
- Two people pointed out that you can avoid MX lookups by putting the
  hostname or IP address in square brackets [exchange.ourdomain.com] in
  sendmail.cf. A quick attempt at this failed for other reasons, so we're
  going to go back and test it a bit more thoroughly. Later.

In the meantime, mail is flowing and we're down to only 3000 backlogged
messages.

Thanks again!
Colin

[Original post]
> This morning our sendmail servers started to misbehave. We're running
> sendmail 8.13.6 on Solaris 9, and sending all inbound mail to a server
> called
> "exchange.ourdomain.com."
>
> Fundamentally, the problem is that sendmail is repeatedly trying to look
> up
> exchange.ourdomain.com in DNS and failing (as it should). Once in a while
> it gets the proper address presumably from /etc/hosts, but then keeps
> going
> back to DNS. Right now, roughly 90% of our email is getting stuck and not
> getting through.
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