test SMTP connection using telnet

How to Test SMTP Communication

If you have problems sending emails over SMTP, you can test the SMTP connectivity by using Telnet on the sending server to connect to port 25 on the destination server. Alternatively, if you are having problems receiving SMTP mail from the Internet, you can follow the steps that are listed in this article to test connectivity to your SMTP server from a host that resides on the Internet and that is not on your network.