MX that points to CNAME instead of A?

[Jim Hickstein]

> Can someone point me to something clearly definitive on the topic?

RFC-974 page 6 has this:

   Note that the algorithm to delete irrelevant RRs breaks if LOCAL has
   a alias and the alias is listed in the MX records for REMOTE.  (E.g.
   REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL).  This
   can be avoided if aliases are never used in the data section of MX
   RRs.

2821 obsoletes 974, though.

My own experience with sendmail (long ago) certainly taught me to avoid
using anything but a canonical name (not an alias) on the RHS of the MX.
 I'm still telling people this
(http://www.emailthatworks.net/forum/read.php?3,35), and I list two more
good reasons to avoid it, but it's not definitive.