"If an alias record (CNAME) is used for the hostname listed in the MX record, the host might re-write the envelope and redirect the RCPT command to the alias hostname and not the original. This might cause the destination SMTP host to reject the message."
The host name contained in an MX record must have an address, i.e. an A or AAAA DNS record "CNAME aliases are prohibited in an MX record data, while CNAME is allowed for the MX record label, that is for the domain name leading to the MX record itself."
- ^ The prohibition on labels in the data that resolve to CNAMEs is discussed in RFC 2181, Section 10.3; its rationale is to allow a backup MX to recognize itself by name, thereby simplifying the algorithm used to determine the target host. See also RFC 1034
"Though the practice of pointing MX records to CNAME (alias) records is not that uncommon, it certainly isn't in keeping with internet standards.
When you point a MX record to a CNAME, you're in fact inviting double the DNS traffic to your DNS servers. Try this by performing a name resolution query using nslookup:
To resolve the CNAME, the sender's DNS server will have to perform a second query. Not only is that inefficient, it is in fact explicitly prohibited by RFC 2181
Exchangepedia Blog: Should MX record point to CNAME records (aliases)?
MX record - Wikipedia, the free encyclopedia