An A record is an MX record and is a missing MX....

Fri Apr 4 02:16:01 UTC 2003

> MX records are only required if you want to have more than one mail
> exchange servers to serve your domain, e.g. if you want to have a
> secondary mail server as a relay if the primary server goes down.

actually, i suspect the more common use is that one has a collector
server for a lot of local infrastructure, as in

   @      A     666.42.7.11
   foo	  A	666.42.7.12
	  MX  0 @
   bar	  A	666.42.7.13
	  MX  0 @

randy