MX lookup
Look up a domain's MX records in priority order, check each host resolves, and see which provider hosts its mail.
An MX lookup lists the servers that accept email for a domain, each with a priority number. Lower numbers are preferred, and servers with the same number share the load (RFC 5321, section 5.1). This lookup also checks that each server resolves to an address, spots a null MX, and names the mail provider when it recognizes the host names.
Last verified . The sources named on this page were read that day.
MX lookup, the tool
Sample report. kestrel-labs.example is a made-up domain
Pass
1 MX host, and it resolves. Mail is hosted by Google Workspace.
Looked up kestrel-labs.example (MX)
| Priority | Host | Addresses |
|---|---|---|
| 1 | smtp.google.com | 64.233.178.2674.125.197.2674.125.197.2764.233.178.272607:f8b0:4020:c07::1a2607:f8b0:4020:c0b::1a2607:f8b0:4020:c0b::1b2607:f8b0:4020:c07::1b |
Recognized from the host names as Google Workspace.
The rest of the report
- SPFFailSPF needs 11 DNS lookups and the limit is 10, so receivers return a permanent error and SPF fails.
- DKIMWarningDKIM key found at google: 1024-bit RSA key. RFC 8301 says signers should use at least 2048 bits.
- DMARCWarningDMARC is at p=none, which makes no request about failing mail. Once reports show all your senders pass, move to quarantine.
- BIMINot foundNo BIMI record. BIMI is optional, and it needs DMARC at quarantine or reject before any inbox shows a logo.
- MTA-STSNot foundNo MTA-STS record. It is optional; it tells senders to refuse delivery without verified TLS to your listed MX hosts.
- BlocklistsCould not checkNot listed on the 14 queries that answered. Spamhaus DBL could not be checked from our server.
Priorities
A sending server sorts your MX hosts by number and works down the list. Replies, bounces and DMARC reports to your domain arrive this way, so if no host resolves, that mail is lost.
10mx1.yourmailhost.exampleTried first. Equal numbers share the load, in random order.10mx2.yourmailhost.example20backup.othermail.exampleTried after the two at 10.
Null MX
0 .- One MX record with preference 0 and a dot for a host says the domain accepts no mail, so senders fail at once instead of retrying. It must be the only MX record.
- Not on a sending domain
- RFC 7505 says not to publish one on a domain used in From or bounce addresses. The checker accepts it only beside an SPF record of
v=spf1 -all. - No MX at all
- Senders then deliver to the domain’s own address, which may not run a mail server.
What the provider name means
Where mail to the domain is received, matched on the host names each provider gives its customers. It says nothing about who sends for the domain.
| Provider | MX hosts | Source |
|---|---|---|
| Google Workspace | smtp.google.com; older setups use hosts starting aspmx | |
| Microsoft 365 | <domain-key>.mail.protection.outlook.com, or <token>.<subdomain>.mx.microsoft with SMTP DANE | Microsoft, DANE |
| Zoho Mail | mx.zoho.com, mx2.zoho.com, mx3.zoho.com; the ending varies by data center | Zoho |
| Fastmail | in1-smtp.messagingengine.com, in2-smtp.messagingengine.com | Fastmail |
| Proton Mail | mail.protonmail.ch, mailsec.protonmail.ch | Proton |
| iCloud+ custom domain | mx01.mail.icloud.com, mx02.mail.icloud.com | Apple |
| Yahoo | Hosts under yahoodns.net | Host name only; no Yahoo page lists them |
| Mimecast, Proofpoint, Barracuda | Hosts under mimecast.com, pphosted.com, barracudanetworks.com | Host name only. Filters, with the mailbox anywhere behind them |
Any other host shows as unrecognized, self-hosted mail included.
Related tools
- Email blacklist checkCheck a domain against three domain blocklists and its mail servers' addresses against three IP blocklists.
- Email domain checkerCheck a domain's SPF, DKIM, DMARC, MX, BIMI, MTA-STS and blocklist status, with the record to publish for each problem.
- SPF record checkerCheck a domain's SPF record and count its DNS lookups through every include against the limit of 10.