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The short answer: the MX record tells other servers where to deliver your domain’s mail. Each MX has a priority (a number) and the name of a server. Senders try the lowest number first. Trouble starts when there are too many MX records, from different providers, or pointing at the wrong place: mail does not vanish all at once, it leaks away bit by bit, which is why it is hard to track down.
How the MX rules work
| Situation | What happens |
| Two MX records, priorities 10 and 20 | Senders try 10. They only use 20 if 10 does not answer. The numbers themselves do not matter, only the order. |
| Two MX records with the same priority | Senders pick either one, in no fixed order. Both must deliver to the same mailboxes. |
| MX records from two different providers (say, your hosting and Google) | Mail splits: some messages go to one, some to the other. It looks like e-mails are lost. |
| No MX at all | By the standard, senders try to deliver to the domain’s own address (its A record). It may work by accident, but do not rely on it. |
| MX 0 . (the null MX) | States outright that the domain accepts no mail (RFC 7505). Senders give up at once and bounce the message. |
| MX pointing at an IP address | Invalid. An MX points at a name, and that name has its own A or AAAA record. |
| MX pointing at a name that is a CNAME | Forbidden by the standard. Many servers accept it anyway, some do not: mail that fails only sometimes. |
Checking and fixing your MX records
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| A backup MX at another provider is rarely worth it today. Senders already keep retrying for a while when your server is down (each server decides how long). And a backup MX that does not know which mailboxes exist accepts spam for made-up addresses and then bounces it to forged senders, which can land your domain on blocklists. |
| A domain that never receives mail (it only has the website) is better protected with the null MX, 0 .. If it never sends either, add an SPF record that says so: creating your SPF record. |
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Mail reaches some mailboxes and not others, or only sometimes? Tell us the domain and an example of a message that did not arrive. Open a support ticket |
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