RFC 2821 says:
If MX records are present, but none of them are usable, this situation
MUST be reported as an error.
This implies that a partially broken RRset is not grounds for rejecting a
When a domain name associated with an MX RR is looked up and the
associated data field obtained, the data field of that response MUST
contain a domain-name. That domain-name, when queried, MUST return
at least one address record (e.g., A or AAAA RR) that gives the IP
address of the SMTP server to which the message should be directed.
Any other response, specifically including a value that will return a
CNAME record when queried, lies outside the scope of this standard.
This implies that it's OK to reject partially broken MX RRsets. Some
deployed software already does this.
f.anthony.n.finch <dot(_at_)dotat(_dot_)at> http://dotat.at/
FAIR ISLE: SOUTHEASTERLY 3 OR 4, INCREASING 5 OR 6 LATER. SLIGHT OR MODERATE.