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MX Lookup

List the mail servers for a domain, confirm they respond, and check whether they offer STARTTLS.

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About the mx lookup

An MX (Mail Exchange) record tells the rest of the internet which servers are responsible for receiving email addressed to a domain, and in what order to try them. This tool looks up the MX records for a domain, resolves each host to its IP addresses, and checks whether the server accepts a connection on port 25, the port used for server-to-server mail delivery.

Priority values control the order senders try servers in: lower numbers are tried first, and equal priorities are treated as an even split for load balancing. A common setup uses 10 for the primary server and 20 for a backup, so the backup only receives mail when the primary is unreachable. The lookup also checks for STARTTLS support, which lets a plain SMTP connection upgrade to an encrypted one during delivery. Without it, mail travels between servers in clear text and can be read or altered in transit; almost every legitimate provider today supports and expects STARTTLS from receiving servers.

A domain with no MX record and no A record at all cannot receive mail. A domain with an A record but no MX record can still receive mail under an old fallback rule some senders honor, but publishing an explicit MX record is the reliable, standards-based way to declare mail servers. Checking both the record and live connectivity together, rather than the DNS entry alone, is what actually confirms mail can be delivered, since a syntactically correct MX record pointing at a server that no longer answers is just as broken as having no record at all.

Common questions

What is an MX record?
A DNS record that names the servers responsible for receiving email for a domain, each with a priority number that controls the order senders try them.
Why do I need more than one MX record?
If the only mail server is down, senders queue and retry, but delivery is delayed until it comes back. A second server on separate infrastructure lets deliveries succeed immediately instead.
What priority should I use?
Lower numbers are tried first. A common pattern is 10 for the primary server and 20 for a backup, so the backup is only used when the primary does not respond.
Why does the tool check STARTTLS?
Without STARTTLS, mail is delivered between servers unencrypted. Almost all modern mail providers expect it, and its absence is a strong signal of a misconfigured or outdated mail server.
What this tool checks (18 rules)
  • email.mx.duplicate-preference — Two MX hosts share the same preference value
  • email.mx.host-is-cname — MX record points at a CNAME
  • email.mx.host-is-ip — MX record points at an IP address instead of a hostname
  • email.mx.host-no-rdns — Mail server address has no reverse DNS name
  • email.mx.host-unresolvable — MX host does not resolve to an address
  • email.mx.no-record — No MX record — this domain cannot receive email
  • email.mx.no-redundancy — Every mail server resolves to the same address
  • email.mx.null-mx — Domain publishes a null MX (receives no mail)
  • email.mx.single-host — Only one mail server host is published
  • email.smtp-tls.banner-malformed — Mail server's SMTP greeting does not conform to the protocol
  • email.smtp-tls.certificate-expired — Mail server's TLS certificate has expired or is not yet valid
  • email.smtp-tls.certificate-name-mismatch — Mail server's certificate does not cover its own hostname
  • email.smtp-tls.certificate-untrusted — Mail server's TLS certificate does not verify
  • email.smtp-tls.no-starttls — Mail server does not offer STARTTLS
  • email.smtp-tls.port-unreachable — Mail server does not accept connections on port 25
  • email.smtp-tls.probe-unavailable — SMTP transport security was not tested
  • email.smtp-tls.starttls-failed — STARTTLS is advertised but the handshake does not complete
  • email.smtp-tls.weak-protocol — Mail server negotiates a deprecated TLS version