What is an MX record?
An MX record names a server that takes email for a domain, with a priority. Senders try the lowest number first and move on when it does not answer. A domain with no MX records gets no mail, or mail goes to its web server and bounces.
Type a domain or a whole email address. The page lists the mail servers, resolves each to its IPv4 and IPv6 addresses, reads each address's reverse DNS (PTR) and checks that the name points back to the same address, which receiving servers look at when your server sends.
Which email provider does a domain use?
| *.mail.protection.outlook.com | Microsoft 365 (Exchange Online) |
|---|---|
| smtp.google.com, aspmx.l.google.com | Google Workspace |
| *.pphosted.com | Proofpoint, a filter in front of the real mailbox host |
| *.mimecast.com | Mimecast, also a filter |
| *.barracudanetworks.com | Barracuda Email Protection |
| *.zoho.com, *.zoho.eu | Zoho Mail |
| *.protonmail.ch | Proton Mail |
| *.mx.cloudflare.net | Cloudflare Email Routing (forwarding) |
When the MX points at a filter such as Proofpoint or Mimecast, the mailboxes are usually still at Microsoft or Google: look at the domain's SPF record to see who sends.
Common MX mistakes
- An MX pointing at an IP address. MX records must name a host; many senders refuse an address.
- An MX pointing at an alias (CNAME). Forbidden by the standard, though most senders cope.
- Old servers left in. An MX for a server that no longer exists makes senders wait or bounce.
- No reverse DNS on your own server. Matters when the same server sends: receivers distrust addresses without a PTR name.
Questions
The lowest number is tried first. Servers with the same number share the load. Higher numbers are backups, used only when the lower ones do not answer.
An MX record with priority 0 and the host "." (RFC 7505). It says the domain takes no mail, so senders bounce messages at once instead of trying for days. Good for domains that only have a website.
The blacklist check asks 22 blacklists about each mail server's address. For Microsoft 365 or Google, the MX servers only receive; your outgoing mail leaves from other addresses the provider manages.