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=...

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    Since that's not the only thing that's broken at/with Yahoo, I can't help to wonder why you don't just do the sensible thing and define an MX record. – Sven May 10 '17 at 19:26
  • Sven, I'm asking the question, because I'd like to understand what is the correct answer: is this my breakage for not having an MX, or Yahoo's breakage for not correctly implementing the RFC? I'm presumably not the only one whose email delivery is broken by this. – Oliver Crow May 10 '17 at 19:36
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    It's Yahoo's issue but IMO that's besides the point. If you want to receive email for the domain in question from Yahoo senders then create an MX record and stop wondering who's to blame. Yahoo's to blame, but that doesn't change the fact that you're not getting email from Yahoo senders. – joeqwerty May 10 '17 at 20:54

The technically correct answer is that Yahoo! is at fault for not following the RFC. The real answer is to go ahead and create the MX record.

  • As a matter of practicality that seems right. It would be nice to know though whether Yahoo is committed to implementing RFCs as they are generally understood. This is hardly a weird edge case. – Oliver Crow May 10 '17 at 19:49

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