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SMTP Tester

Test SMTP server connections, STARTTLS and SSL/TLS, authentication, and email delivery. Inspect the full SMTP conversation, server capabilities, and certificate details.

Server Settings

Credentials are sent to the SMTP server you specify, held in memory for the test only, and never stored or logged. Use an app password where possible.

Allow invalid certificates

Useful for self-signed or expired certificates

Test Type

Test Results

Configure your server and run a test to see results.

SMTP Conversation

The server conversation will appear here.

About SMTP Tester

SMTP Tester is a free online tool for diagnosing email delivery problems. Connect to any public SMTP server, verify TLS, check authentication, and send a test message while watching the complete SMTP conversation between the tester and your mail server.

Key Features

  • Connection Testing: Verify the server greeting, EHLO response, and advertised extensions
  • STARTTLS & SSL/TLS: Test implicit TLS (port 465) and STARTTLS upgrades (port 587/25)
  • Authentication Check: Validate credentials using AUTH PLAIN or AUTH LOGIN
  • Send Test Email: Deliver a real message to confirm end-to-end delivery
  • Certificate Details: Inspect TLS version, cipher, issuer, and expiry date
  • Full Protocol Log: See every command and reply with per-step timing
  • Provider Presets: One-click settings for Gmail, Outlook, SES, SendGrid, Mailgun, and Zoho
  • Export Results: Download the test report as JSON for documentation or support tickets

How to Use

  1. Pick a provider preset or enter your SMTP host, port, and security mode
  2. Choose a test type: connection, authentication, or send email
  3. Add credentials (and sender/recipient for a send test)
  4. Click "Run Test" and review the steps, TLS details, and conversation log

Common SMTP Ports

  • 25: Server-to-server relay. Often blocked by cloud and residential providers
  • 465: SMTP over implicit SSL/TLS
  • 587: Mail submission with STARTTLS (recommended for apps)
  • 2525: Alternative submission port offered by many providers when 587 is blocked

Troubleshooting Tips

  • Connection timed out: The port is blocked by a firewall or the host is wrong. Try 587 or 2525
  • 535 Authentication failed: Wrong credentials. Gmail and Outlook usually require an app password
  • STARTTLS not advertised: Use SSL/TLS on port 465 or check your server's TLS configuration
  • Certificate not trusted: The certificate is self-signed, expired, or does not match the host name
  • Sender rejected: Many providers require the From address to be a verified sender or domain

Privacy & Limits

Credentials are used only for the duration of a single test and are masked in the conversation log. For safety, connections to private or internal IP addresses are blocked, only standard SMTP ports are supported, and send tests are limited to 5 recipients.