What is an MX record?
An MX (mail exchanger) record is a DNS record that names the server responsible for receiving email for a domain. Each MX record has a priority number and a hostname. Sending servers look the records up, try the lowest number first, and fall back to the next if it does not answer.
How MX priority works
The number in front of each MX host is its preference. Lower means tried first. A sender connects to the lowest-numbered host, and only moves to a higher number if that one is unreachable. Hosts with the same number share the load at random.
example.com. MX 1 aspmx.l.google.com.
example.com. MX 5 alt1.aspmx.l.google.com.
example.com. MX 5 alt2.aspmx.l.google.com.Here mail goes to the first host. If it is down, senders pick either of the two hosts at priority 5. The numbers only matter relative to each other — 1, 5, 10 behaves exactly like 10, 50, 100.
How to read the results
| Finding | What it means |
|---|---|
| MX records found, all resolve | Mail delivery is set up correctly at the DNS level. |
| No MX records | Senders fall back to the domain's A record. Mail usually bounces unless a mail server runs there. |
Null MX (0 .) | The domain has declared that it accepts no email. Intentional, and correct for parked domains. |
| MX host does not resolve | The hostname has no A or AAAA record. Mail to that host fails; if it is the only one, all mail bounces. |
| MX host is a CNAME | Not allowed by the mail standards. Some senders cope, others defer or reject. |
| MX points to an IP address | Invalid. An MX record must contain a hostname. |
| Only one MX host | Not an error — large providers publish one name backed by many servers — but a single self-hosted server has no fallback. |
When the hostnames belong to a well-known provider, the tool names it, which is a quick way to confirm where a domain's mailboxes really live.
How to fix common MX problems
Mail bounces with "no MX" or "host not found". Add the MX records your mail provider lists. Enter them exactly, including the trailing dot if your DNS host asks for fully qualified names — a missing dot can turn mx.provider.com into mx.provider.com.example.com.
The MX host is a CNAME. Point the MX record at the final hostname the CNAME leads to, which must have its own A or AAAA record:
; wrong
example.com. MX 10 mail.example.com.
mail.example.com. CNAME mailhost.provider.net.
; right
example.com. MX 10 mailhost.provider.net.Old provider's records are still there after a migration. Remove them. A leftover MX with a lower number keeps receiving your mail at the old provider.
A domain that should receive no email. Publish a null MX so senders fail immediately instead of retrying for days:
example.com. MX 0 .MX records for common providers
| Provider | MX host pattern |
|---|---|
| Google Workspace | smtp.google.com (or the older aspmx.l.google.com set) |
| Microsoft 365 | yourdomain-com.mail.protection.outlook.com |
| Zoho Mail | mx.zoho.com, mx2.zoho.com, mx3.zoho.com |
| Fastmail | in1-smtp.messagingengine.com, in2-smtp.messagingengine.com |
| Proton Mail | mail.protonmail.ch, mailsec.protonmail.ch |
Always copy the values from your provider's own setup page. Some change by region or by account.