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SMTP Tester - Test SMTP Server Connectivity Online

SMTP Tester

Common SMTP ports. Port 587 is recommended for submission.
What is SMTP Testing?

SMTP (Simple Mail Transfer Protocol) testing verifies that a mail server is accepting connections on a specific port. This tool helps diagnose email delivery problems by testing connectivity to SMTP servers.

How This Tool Works
  1. Enter the SMTP server hostname or IP address
  2. Select the port to test (25, 465, or 587)
  3. The tool attempts to connect to the server
  4. If successful, it displays the server greeting and response time
  5. If unsuccessful, it shows the error message to help diagnose issues
SMTP Ports Explained
  • Port 25: Standard SMTP port for server-to-server communication. Often blocked by ISPs.
  • Port 465: SMTP with implicit SSL/TLS. Legacy but still widely used.
  • Port 587: SMTP submission port with STARTTLS. Recommended for email clients.
  • Port 2525: Alternative SMTP port used when others are blocked.
Common Test Results
Successful Connection: 220 smtp.example.com ESMTP Postfix Response Time: 45 ms Connection Refused: The server is not accepting connections on this port. Possible causes: wrong port, firewall, or server down. Timeout: No response from server after 10 seconds. Possible causes: wrong hostname, network issues, or firewall blocking.
Understanding Server Greetings

The SMTP greeting provides information about the mail server:

  • 220: Service ready (successful connection)
  • ESMTP: Extended SMTP with additional features
  • Server Name: Often reveals the mail server software (Postfix, Exim, Exchange, etc.)
Common Connection Issues
  • Connection Timeout: Wrong hostname, network/firewall blocking, or server offline
  • Connection Refused: Server not listening on that port, or port closed
  • Port 25 Blocked: Many ISPs block outbound port 25 to prevent spam
  • DNS Resolution Failed: Hostname doesn't exist or DNS issues
  • SSL/TLS Errors: Certificate issues on ports 465/587
Troubleshooting Steps
  1. Verify Hostname: Ensure you're using the correct mail server address
  2. Try Different Ports: If one port fails, test others (especially 587)
  3. Check Firewall: Ensure your firewall allows outbound connections
  4. Test from Different Networks: Some ISPs block SMTP ports
  5. Verify MX Records: Use MX Checker to find correct mail servers
  6. Check Server Status: Ensure the mail server is running and accessible
Example: Testing Popular Email Providers
Gmail (Google Workspace): Host: smtp.gmail.com Port: 587 (TLS) or 465 (SSL) Microsoft 365/Outlook: Host: smtp.office365.com Port: 587 (TLS) Yahoo Mail: Host: smtp.mail.yahoo.com Port: 587 (TLS) or 465 (SSL) SendGrid: Host: smtp.sendgrid.net Port: 587 (TLS) or 465 (SSL)
Security Considerations
  • This tool only tests connectivity, it does NOT authenticate or send emails
  • No credentials are transmitted or stored
  • Port 587 with STARTTLS is the most secure modern option
  • Port 465 uses implicit SSL (also secure)
  • Port 25 should only be used for server-to-server communication
Best Practices
  • Use port 587 with STARTTLS for email client connections
  • Ensure SMTP server has valid SSL/TLS certificates
  • Implement rate limiting to prevent abuse
  • Use authentication for all mail submissions
  • Configure SPF, DKIM, and DMARC for sender authentication
  • Monitor SMTP logs for connection issues and abuse
  • Keep mail server software updated for security
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