What are POP and SMTP servers?

Post Office Protocol is the most
common protocol used to
retrieve e-mail from a mail server.
Most e-mail applications
(sometimes called an e-mail
client) use the POP protocol,
although some can use the
newer IMAP (Internet Message
Access Protocol). The newest
version, POP3, can be used with
or without SMTP (an e-mail
sending protocol, stands for
Simple Mail Transfer Protocol).
IMAP servers are similar to POP
servers, the only difference being
they save the e-mail so they can
be retrieved from multiple
locations or multiple users.