SMTP Test

SMTP Test: diagnose mail server connectivity online

SMTP (Simple Mail Transfer Protocol) is the standard protocol for transferring email between servers. This tool connects to the domain's MX server and walks through the handshake sequence — banner, EHLO, STARTTLS — letting you diagnose mail delivery problems without any mail client.

What does the test check?

• MX record — DNS record pointing to the domain's mail server
• TCP connection — port 25 (SMTP) or 587 (SUBMISSION) reachability
• SMTP banner (220) — server greeting message
• EHLO — ESMTP extension capabilities
• STARTTLS — in-session TLS encryption support

Frequently asked questions

• Why is port 25 blocked?

  Most hosting providers block outbound port 25 to prevent spam. Use port 587 (SUBMISSION) with SMTP authentication for sending, or 465 (SMTPS) for implicit TLS.

• What is STARTTLS?

  STARTTLS upgrades an existing plain-text SMTP connection to TLS encryption mid-session. This differs from SMTPS (port 465) where TLS is established from the first byte. STARTTLS is the recommended standard for server-to-server email encryption.

• How do I test SMTP manually?

  Run telnet mx.example.com 25 in a terminal. After the 220 banner type EHLO yourdomain.com, then QUIT. On Windows use PuTTY or Test-NetConnection -ComputerName mx.example.com -Port 25.

See also

• MX Records — mail server lookup for any domain
• DKIM Checker — verify DKIM signature
• Email Security — SPF, DKIM, DMARC graded A–F