How To's

How to Use Telnet to Test SMTP Communication

Telnet is an extremely useful tool for troubleshooting issues related to SMTP and mail flow

For example, you can use telnet to:

  • Verify that SMTP is installed properly, and that it has all the necessary commands.
  • Ensure that your server is accessible over the Internet.
  • Attempt mail delivery directly over the TCP port.
  • Determine that all servers are accepting connections.
  • Determine if a firewall is blocking a connection.
  • Ensure that a single user can receive mail.
  • Ensure that a specific domain can receive mail.
  • Ensure that a specific user or domain can send mail to your domain.


To use telnet to test SMTP communication

  1. Open a telnet session: From a command prompt, type telnet, and then press ENTER
  2. Type o <your mail server domain> 25,and then press ENTER.
  3. Type EHLO <your mail server domain>, and then press ENTER.
  4. Type AUTH LOGIN. The server responds with an encrypted prompt for your user name.
  5. Enter your user name encrypted in base 64. You can use one of several tools that are available to encode your user name.
  6. The server responds with an encrypted base 64 prompt for your password. Enter your password encrypted in base 64.
  7. Type MAIL FROM:<>, and then press ENTER. If the sender is not permitted to send mail, the SMTP server returns an error.
  8. Type RCPT TO:<>,and then press ENTER.If the recipient is not a valid recipient or the server does not accept mail for this domain, the SMTP server returns an error.
  9. Type DATA.
  10. If desired, type message text, press ENTER, type a period (.), and then press ENTER again.
  11. If mail is working properly, you should see a response similar to the following indicating that mail is queued for delivery:
    250 2.6.0 <

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