question about CNAME and MX

[Jonathan de Boyne Pollard]

MU> The problem can be, that any @comceptsolutions.com destination
MU> address @would be rewritten by sender MTA's (if they follow the
MU> RFC) to @comcept.net, if he used "comceptsolutions.com. CNAME
MU> comcept.net.

Leaving aside the issue with those DNS data themselves:  Any SMTP Relay client
rewriting mailbox names in that fashion is out of date.  The notion of SMTP
Relay clients rewriting mailbox names by chasing client-side DNS aliases was
finally quashed in the SMTP world some 3 years ago.  The SMTP Relay clients
that "follow the RFC" will _not_ operate as you describe.