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Background: CentOS 8, Postfix, Dovecot, Digital Ocean droplet.

My mail server can send and receive emails without a problem. My DNS records include an A, AAAA and MX record for my mail server.

A mail.example.com     directs to 10.10.10.1
AAAA mail.example.com     directs to 1111:1111:1111:1111:1111:1111:1111
MX mail.example.com     mail handled by mail.example.com

However when I tested my setup on MXtoolbox the MX Lookup test returned an error.

Category: MX        Result: DNS Record not found

Question) Which record is missing or misconfigured? I'm not sure what to add or repair in this instance especially since emails can be sent and received.

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    Unless your domain is example.com, we can't help you. Now, if you'll share the actual domain with us we can tell you what we see. – joeqwerty Apr 27 '20 at 20:53
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Your domain does not have an MX record. But an MTA is listening on the A record's IP, and that is the fallback of SMTP in case there is no MX record.

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    You were right. My MX record was setup but I had done it wrong. It should have been Type: MX Hostname: example.com Value: mail handled by mail.example.com – myNewAccount Apr 27 '20 at 21:17
  • The original design was the other way around: MX was an override of the A record. Originally MX was for exceptional cases, such as relaying to servers on other networks like UUCP or BITNET. It has since become the expected case. – Barmar Apr 28 '20 at 13:49

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