SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol
used in sending and receiving e-mail. However, since it's limited in its
ability to queue messages at the receiving end, it's usually used with
one of two other protocols, POP3 or Internet Message Access Protocol,
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 for receiving
messages that have been received for them at their local server. Most
mail programs such as Eudora let you specify both an SMTP server and a
POP server. On Unix-based systems, sendmail is the most widely-used SMTP
server for e-mail. A commercial package, Sendmail, includes a POP3 server
and also comes in a version for Windows NT.
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