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Checker

SMTP test and TLS check for mail servers

Test a mail server on port 25: connection, greeting, offered extensions, STARTTLS, TLS version, certificate and reverse DNS, without sending an email.

What the SMTP test covers

  • Connection: whether the server answers on port 25 and how fast.
  • Greeting and extensions: what the server reports, such as maximum message size and login methods.
  • STARTTLS and certificate: whether encryption is offered, with which TLS version and whether the certificate is valid and matches the name.
  • Reverse DNS: whether the server IP has a matching hostname.

We do not send an email and log off right after the encryption handshake. Only publicly reachable servers are tested. MX, STARTTLS and MTA-STS checker

FAQ

What does the SMTP test check?

We connect to port 25 of the mail server and read the greeting and extensions. Then we check STARTTLS, the TLS version, the certificate and the reverse DNS record. No email is sent.

Why is port 25 not reachable?

Common reasons are a firewall, a provider that blocks port 25, or a server that accepts email only from known senders. Our tests come from a data center in Germany.

Can I also enter a domain?

Yes. If the domain has no address of its own, we test the first mail server from the MX records.