MX lookup - mail server records

What an MX record is

The MX record tells the rest of the internet which server(s) to hand email to for a domain. Each one carries a priority number - lower is tried first - so a domain can list a primary mail server and one or more fallbacks.

Reading the priority

A sending server tries the lowest-numbered MX first and works upward only if that one is unreachable. Several records sharing the same lowest number means the sender may pick any of them - a simple form of load balancing.

Why this is the first thing to check for email problems

If MX records point at the wrong server, or are missing entirely, mail cannot be delivered no matter how correctly everything else (SPF, DKIM, the mail server itself) is configured - it is the address the letter is mailed to, checked before any of the sender-authentication rules a receiving server applies afterward.

Related tools

Once mail is reaching the right server, check its authentication with SPF, DKIM and DMARC.