sendmail finds multiple A or AAAA records for a
host (and no MX records), it tries them in the order returned by DNS,
but looks up and uses AAAA before A records. If
sortlist is specified in the
/etc/resolv.conf file, DNS returns the A or AAAA
record that is on the same network first. The
sendmail program assumes that DNS returns
addresses in a useful order. If the address that
sendmail always tries first is not the most
appropriate, look for problems with DNS, not with
If you misunderstand the
(TryNullMXList) and mistakenly set it to true under
the wrong circumstances, you might one day suddenly discover many
queued messages from outside your site destined for some host
you’ve never heard of before.
Under old versions of DNS an error in the zone file causes the rest of the file to be ignored. The effect is as though many of your hosts suddenly disappeared. This problem has been fixed in 4.8.3 and 4.9.x.
Sites with a central mail hub should give that hub the role of a
caching secondary DNS server. If
/etc/resolv.conf contains the address of
localhost as its first record, lookups will be
much faster. Failure to make the mail hub any sort of DNS server runs
the risk of mail failing and queueing when the hub is up but the
other DNS servers are down or unreachable.
Prior to V8.8 sendmail the maximum number of MX records that could be listed for a single host was 20. Some sites, such as aol.com, might reach that limit soon and exceed it. ...