You need no MX record to get mails

That one is funny. One client of ours that is actually themselves provide ISP services in a far-far-away land asked to add PTR record for their mail server . But that was dull, the interesting part was that their domain had absolutely NO MX record ! Only A record for the mail server host . I had always thought if there is no MX record for the destination domain sending mail server should bail out and I was wrong. A SMTP RFC 5321 actually states that if there no MX record exists for the domain the sender should try delivering the mail to A record of the domain RFC 5321 section 5 . Be aware though that MX record should be completely absent, so say if MX record does exist but points to a not responding server is a different case - in such case sender should fail the delivery.
The funny thing about that is that they have been working without MX record for about 2 years and have had no problems with receiving the mails, just amazing how  RFC-compliant mail servers in the wild are.

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