Simple Mail Transfer Protocol

SMTP is the protocol that email servers use to forward messages across a TCP/IP network. The client computer that initiates an email message also uses SMTP to send that message to a local server for delivery.

A user never has to speak SMTP. The SMTP communication process goes on behind the scenes. However, it is occasionally important to know a little about SMTP to interpret error messages for undelivered mail. Also, programs and scripts sometimes access SMTP directly to send email warnings and alerts to network personnel.