When the first domain record is a CNAME, why are MX records not allowed?
RFC 1034 published by the Internet Engineering Task Force (IETF) recommends the following:
"If a CNAME resource record is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different. This rule also insures that a cached CNAME can be used without checking with an authoritative server for other resource record types."
We do have a workaround for this issue.