MX Mail Exchanger

The record saying which server takes delivery of mail for your domain. Each record carries a number in front of it, the priority. The lower one wins, the others are fallbacks.

An MX always points at a hostname, never at an IP address. And that hostname has to resolve in its own right. If it points at a host that no longer exists, the record still looks valid and still takes no mail.

With no MX at all, some senders fall back on the domain's A record. It is not something to rely on.