Telnet allows you to manually start an SMTP session and carry out the following diagnosis:
- Check implementation of SMTP server
- Check the accessibility of all required SMTP commands
- Detect connection blockages caused by a firewall or anti-virus program
- Ensuring that a user or a particular provider domain is receiving your message
The following example demonstrates how to test the SMTP server connection from an internal client to a server using basic authentication on Windows:
1. Open the command prompt (search the term "cmd" in the search bar)
2. Enter the “telnet smtp.example.com 25” command to connect to the SMTP server via port 25 (Replace "smtp.example.com" with the address of your own SMTP server.)
3. If the server is reachable, it replies back with status codes 220 and "smtp.example.com ESMTP Postfix", or a comparable plain text message. This would indicate that the SMTP server has no connection error.
You can then authenticate and, if necessary, send a test e-mail to pinpoint the cause of the problem. If the e-mail doesn’t reach its destination after the connection has been proven as working, the issue most likely lies with your provider or the recipient.
If there’s no answer or an error message being returned from the server, your firewall or antivirus program could be one possible cause of the e-mail transmission problem. Alternatively, you can also test the SMTP server connection using an online tool such as wormly.