Yahoo's Mail servers are refusing to deliver messages to the mail server for a domain I manage. It complains that there is no MX record for the domain. According to my understanding for many years, and according to RFC 5321, if there is no MX record, mail servers are expected to fall back to the address specified in the A record for the domain. Here's the relevant sentence from the RFC:
Section 5.1 "If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host."
It seems I'm not the only one to have noticed this apparent breakage of RFC compliance by Yahoo's mail delivery servers. My question is: must site operators now create MX records for all domains that receive mail? Is there some process available for reporting to Yahoo that their mail servers are non-compliant? Or am I misinformed, and it is no longer required for mail servers to fallback to A records when MX records are not present?
This is the top of the bounce message from Yahoo, with addresses removed:
Sorry, we were unable to deliver your message to the following address. <...>: No mx record found for domain=...