MX records for mail delivery

An MX record tells the rest of the internet which server accepts mail for your domain. No MX record means no incoming mail at all, so if Mail Check reports this one it is the first thing to fix.

bash

dig MX example.com +short
# 10 mail.example.com.
# 20 backup.example.com.

Priorities

The number in front is the priority and lower wins. Senders try the lowest number first and move down the list if it does not answer. The numbers themselves do not matter, only their order, so leave gaps like 10 and 20 to give yourself room to insert something later.

Publish a second one

With a single MX, any outage means senders get a temporary failure and start retrying. Most will keep trying for a few days so mail is rarely lost, but it sits in other people queues where you cannot see it. A second MX gives it somewhere to land.

Point MX at a name, not an address

An MX record has to contain a hostname. Putting an IP address there is invalid and some senders will refuse it outright. Publish an A record for the mail host and point the MX at that name.

Also make sure the name in the MX actually resolves. A record pointing at a host that no longer has an A record fails in a way that is easy to miss, because the MX lookup itself still succeeds.