Simple Mail Transfer Protocol (SMTP), is the Internet protocol for electronic mail delivery that most mail servers today use to deliver e-mail. Whenever you send an e-mail, your e-mail client interacts with the SMTP server. The SMTP server on your host may communicate with other SMTP servers to actually deliver the e-mail.
How it works
During an SMTP monitoring task the agents monitor the SMTP server by connecting to the specified mail server and performing an SMTP handshake. If a remote server does not respond to a handshake, or is unavailable, an error condition will be triggered and if you have set up alerting, you will receive an alert.