How to monitor SMTP servers
To monitor SMTP servers simply add a test to Netmon with the test type ‘SMTP’. Make sure your port is set at 25, or set the the appropriate port number if you run your SMTP on a non-standard port.   If you just do the above, then Netmon will alert you when the SMTP port is no longer alive. However, there are many occasions when the SMTP port is alive, but the service cannot respond correctly. For instance, the service may be alive, but the server run out of disk space or suffering another problem. In that type of situation the SMTP server will respond with an error code other than the ‘220’ everything is OK code. To check your SMTP status more thoroughly you can add ‘220’ to the ‘check response for’ field. This will ensure that your SMTP server is not only alive, but isn’t suffering some other issue that will prevent it accepting email.