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SMTP Connectivity Test

Test SMTP connectivity, TLS and authentication to any mail server. Optionally send a test email after a successful connection.

SMTP Configuration

Enter your SMTP server details

Common: 25, 465, 587

About SMTP Testing

SMTP (Simple Mail Transfer Protocol) is the protocol used to send email between servers. This tool establishes a real SMTP connection to your mail server, performs the handshake sequence, and optionally authenticates and sends a test message — showing the full SMTP conversation log so you can diagnose exactly where a problem occurs.

Common SMTP ports

  • Port 25 — Server-to-server mail transfer (MTA). Often blocked by ISPs on residential connections to prevent spam. Used for direct delivery between mail servers.
  • Port 465 — SMTPS (implicit SSL). The original secure SMTP port, deprecated and then revived. Uses SSL from the start of the connection.
  • Port 587 — Mail submission port. The recommended port for email clients and applications to submit mail to a server. Uses STARTTLS to upgrade to an encrypted connection.
  • Port 2525 — Unofficial alternative to 587 used by some providers (e.g. Mailgun, SendGrid) when 587 is blocked.

Reading the SMTP log

Lines starting with C: are commands sent by this tool; lines starting with S: are responses from the server. Key response codes:

  • 220 — Server ready (banner)
  • 250 — Command accepted (OK)
  • 235 — Authentication successful
  • 334 — Challenge (server awaiting authentication credential)
  • 354 — Start mail input (ready to receive message data)
  • 421 / 450 / 550 — Errors (temporary or permanent rejection)
  • 535 — Authentication failed (wrong credentials)

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Frequently asked questions

What SMTP ports should I use?
Port 587 (Submission + STARTTLS) is the standard for client sending. Port 465 uses implicit TLS (SMTPS). Port 25 is server-to-server and is often blocked by ISPs. Port 2525 is an alternate submission port some providers use.
Is my password sent securely?
Your credentials are used only to perform the AUTH test against the server you specify. They are not stored, logged, or transmitted anywhere else. On TLS-capable ports the connection is encrypted before authentication is attempted.
What does the Security setting do?
Auto detects the right mode based on port (465 = implicit SSL, 587/2525 = STARTTLS, 25 = plain). SSL forces implicit TLS from the start. TLS forces STARTTLS and fails if unavailable. "TLS when available" tries STARTTLS but continues if the server does not advertise it. None skips TLS entirely.
What does AUTH LOGIN vs AUTH PLAIN mean?
Both are standard SMTP authentication mechanisms. AUTH PLAIN sends credentials in one Base64-encoded step. AUTH LOGIN sends them in two separate exchanges. Most mail servers support both; PLAIN is preferred where available.