Sendmail, 3rd Edition

Pitfalls

• When 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 sendmail.

• If you misunderstand the TryNullMXList option (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.^[16]

• 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. ...