Can MX record be CNAME?

[Peter Peltonen]

I have a mail server with A record a.b.c and PTR record 1.2.3.4. The MX record
says naturally a.b.c too.

I want to change the servers dns name to mail.b.c.

Unfortunately I do not control my PTR records so I have to do the dns name
change with CNAME. 

My questions are:

Can MX record point to a CNAME?

Or maybe I should use multiple A records?

In both cases, what do I put in /var/qmail/control/me? The same name as in the
MX record?

Peter