- 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.