EMAIL DIAGNOSTICS
Test a public SMTP server’s port 25 response, advertised ESMTP capabilities and recipient-stage relay policy.
Connecting to the SMTP server and checking its capabilities. Please wait.
The test connects only to port 25. It does not authenticate, send message content or issue a DATA command. Checks are limited to 3 per visitor per hour and 3 per server per day.
The checker opens one TCP connection to the server’s public port 25, reads its SMTP greeting, sends EHLO and reports advertised capabilities such as STARTTLS, SIZE and AUTH.
For the relay check it submits a null sender and a recipient under the reserved .invalid domain. It never sends DATA or message content. Recipient acceptance is only a potential relay signal; some servers make their final routing decision later.
An open relay is an SMTP server that accepts mail from an unauthenticated sender and forwards it to unrelated external recipients. Such servers are commonly abused for spam.
Relevant standard: RFC 5321 — Simple Mail Transfer Protocol
No. The test stops at the RCPT command and then resets the SMTP transaction. It never sends the DATA command, a message body or an authentication attempt.
Relevant standard: RFC 5321 — Simple Mail Transfer Protocol
STARTTLS is an SMTP extension that upgrades an existing plaintext SMTP connection to TLS. Its presence alone does not prove that a server enforces encryption or has a valid certificate.
Relevant standard: RFC 3207 — SMTP Service Extension for Secure SMTP over TLS
SMTP servers can defer recipient validation and routing decisions until later in their processing pipeline. A recipient accepted during RCPT needs follow-up investigation, but the tool does not deliver a message.
Relevant standard: RFC 5321 — Simple Mail Transfer Protocol