What is an MX record?

An MX record tells a sending server where to deliver mail for your domain. Each entry pairs a priority with a hostname; the sender tries the lowest number first and works upward, and equal numbers are load-balanced between.

How delivery resolves

A sender queries MX for the recipient's domain, picks the lowest-priority host, resolves that hostname to an A or AAAA record, and connects to port 25 there. Every step has to work: an MX host with no address record is a dead end, and the sender falls through to the next entry or queues the message.

With no MX at all, RFC 5321 §5.1 says a sender falls back to the A record of the domain itself — which is usually your web server, which usually does not accept mail. That fallback is why a missing MX often looks like silence rather than an error.

Two rules that are regularly broken

An MX must point at a hostname with an address record, never at a CNAME. RFC 2181 §10.3 forbids it, and while some senders follow the alias anyway, others do not — which produces delivery that works from one sender and fails from another.

A null MX (0 ., RFC 7505) is a deliberate statement that the domain accepts no mail. It is the correct configuration for a domain you only send from, or do not use for mail at all, and it lets receivers reject immediately instead of timing out.

MX and SPF are separate

MX controls incoming mail only. Which hosts may send for your domain is the SPF record, and the two lists are frequently different: mail arrives at Microsoft 365 while the newsletters go out through a marketing platform. Changing MX does not change who may send as you.

The scan at the bottom of this page resolves every MX host, reports which have no address record and which are CNAMEs, and recognises the mail platform behind them.

Frequently asked questions

Do I need a backup MX?

Rarely. Sending servers already queue and retry for days when the primary is unreachable, so a backup MX mostly adds a host that has to be kept as well secured and as well filtered as the primary — and it is a favourite target precisely because it usually is not.

Two MX records point at different platforms. Is that a problem?

Almost always yes. It means mail is split unpredictably between two mailstores depending on which host a sender reached, which is the classic signature of a migration that was never finished.