Free MX lookup

How a sending server uses the list

When a server wants to deliver mail to [email protected], it asks for the MX records of example.com. Each record has a priority number and a host name, and the sender tries them from the lowest number upwards. Records with the same number are shared between: a domain with two entries at 10 receives roughly half its mail on each. The result is printed as the domain, the TTL, "MX" and then "priority host", for example 10 mail.example.com.

If a domain has no MX at all, the standard says to fall back to its A or AAAA record as an implicit mail host. That is rarely what anyone intends, and a typo in a domain name often turns into mail delivered to a web server. To declare that a domain accepts no mail, publish a null MX: a single record 0 . (a dot), defined in RFC 7505.

What the common providers look like

Recognising a provider's pattern makes a migration check quick:

• Google Workspace, classic set: 1 aspmx.l.google.com, 5 alt1.aspmx.l.google.com, 5 alt2.aspmx.l.google.com, 10 alt3.aspmx.l.google.com and 10 alt4.aspmx.l.google.com. Google also supports a single 1 smtp.google.com record for new setups.
• Microsoft 365: one record, priority 0, in the form yourdomain-com.mail.protection.outlook.com, where dots in the domain become hyphens.
• Self-hosted or hosting-panel mail: usually 10 mail.yourdomain.com with an optional 20 backup. A backup host must run the same spam filtering as the primary, or it becomes the easy way in for junk and for bounce messages sent to forged senders.

Rules MX records must follow

The target of an MX has to be a host name with its own A or AAAA record. It may not be an IP address and, under RFC 2181, it should not be a CNAME, although some software tolerates one. A trailing dot in the output, as in mail.example.com., marks a fully qualified name. If the host listed here has no address, the lookup of that host is the next thing to run: use the A lookup on it, and the PTR lookup on the sending IPs when mail goes out.

The record only routes inbound mail. Whether a receiver trusts your outbound messages depends on SPF, DKIM and DMARC, which are TXT records with their own tools.

Changing mail providers without losing messages

Lower the TTL of the MX records to 300 seconds a day or more before the switch, set up mailboxes at the new provider first, and then replace the records in one edit. Do not mix old and new hosts in the same set, because senders will spread messages between them at random. Senders do retry: Postfix, for instance, keeps a deferred message for up to five days by default. That protects you from a short gap, but it does not help if the wrong server accepts a message and discards it. After the change, run this lookup, send a test from an outside address, and wait for the old TTL to expire before decommissioning the old mailboxes.

Watching mail routing

Mail that stops arriving is quieter than a website that fails, and the first complaint often comes days later. A DNS monitor on the MX record with the expected hosts warns you when the set changes or disappears, and matches on host name alone or on priority plus host. Pair it with a port monitor on the mail host to see when the machine stops accepting connections. The port monitor works on Free, which has 30 monitors at 5 min; the DNS monitor starts on Starter at $7 a month, which also shortens the interval to 1 min.