POP3 (Post Office Protocol 3) is the most recent version of a standard protocol for
receiving e-mail. POP3 is a client/server protocol in which e-mail is received
and held for you by your Internet server. Periodically, you (or your client e-mail receiver)
 check your mail-box on the server and download any mail. POP3 is built into the Netmanage
suite of Internet products and one of the most popular e-mail products,
Eudora. It's also built into the Netscape and Microsoft Internet Explorer browsers.

An alternative protocol is IMAP (Internet Message Access Protocol). With IMAP,
you view your e-mail at the server as though it was on your client computer.
An e-mail message deleted locally is still on the server. E-mail can be kept on and
searched at the server.

POP can be thought of as a "store-and-forward" service. IMAP can be thought of as a
remote file server.

POP and IMAP deal with the receiving of e-mail and are not to be confused with the
Simple Mail Transfer Protocol (SMTP), a protocol for transferring e-mail across the
Internet. You send e-mail with SMTP and a mail handler receives it on your recipient's
behalf. Then the mail is read using POP or IMAP.

IMAP (Internet Message Access Protocol) is a standard protocol for accessing e-mail from
your local server. IMAP (the latest version is IMAP4) is a client/server protocol in which
e-mail is received and held for you by your Internet server. You (or your e-mail client)
can view just the heading and the sender of the letter and then decide whether to download
the mail. You can also create and manipulate folders or mailboxes on the server,
delete messages, or search for certain parts or an entire note. IMAP requires continual
access to the server during the time that you are working with your mail.

A less sophisticated protocol is POP3 (Post Office Protocol 3). With POP3, your mail is
saved for you in your mail box on the server. When you read your mail, all of it is
immediately downloaded to your computer and no longer maintained on the server.

IMAP can be thought of as a remote file server. POP can be thought of as a
"store-and-forward" service.

POP and IMAP deal with the receiving of e-mail from your local server and are not to
be confused with SMTP, a protocol for transferring e-mail between points on the Internet.
You send e-mail with SMTP and a mail handler receives it on your recipient's behalf.
Then the mail is read using POP or IMAP.

SMTP 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 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 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.

SMTP usually is implemented to operate over TCP port 25
The details of SMTP are in RFC 821 of the Internet Engineering Task Force (IETF).
An alternative to SMTP that is widely used in Europe is X.400.