SMTP (Simple Mail Transfer Protocol) is a TCP/IP
protocol used in sending and receiving e-mail.
However, since it is limited in its ability to queue
messages at the receiving end, it is usually used
with one of two other protocols, POP3 or IMAP,
that let the user save messages in a server
mailbox and download them periodically from the
server. In other words, users typically use a
program that uses SMTP for sending e-mail and
either POP3 or IMAP...