Solve MX issues

wikipedia.org:
https://en.wikipedia.org/wiki/MX_record

Common failures:

  • The domain name itself is in MX: The mail server of e.g. shared hosting server has a different name on the SSL certificate;
  • The web server itself may take care of incoming email. Depending on mail server setup, SPF must authorize for bounces;
  • For ‘www.’ MX can be set up like ‘0 .’. Otherwise, A/AAAA values are used. For explanation, see ‘Null MX’, RFC 7505.

Note on October 1, 2020: Control Panel DirectAdmin refuses MX ‘0.’ setup.