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SMTP
(Simple Mail Transfer Protocol) enables computers to send
mail to each other via the Internet. SMTP pertains only
to the protocol used by computers to transfer and deliver
e-mail.
POP (Post Office
Protocol) enables mail recipients to retrieve mail that
has arrived.
IMAP (Internet
Message Access Protocol) enables message retrieval and
storage.
SMTP Server
In order to send and
receive e-mail across the Internet, an SMTP server must
meet the following requirements:
- Should have a continuous
Internet connection and be prepared to receive mail
at all times because incoming mail can arrive at any
time of day or night.
- Should be able to
deliver outgoing messages on behalf of a computer
that does not have complete SMTP capabilities.
- Should be able to
perform relays on behalf of other computers. When
an SMTP server is asked to deliver a message on behalf
of another computer, and the recipient of the message
is not a local user on the system, then the SMTP server
should relay the message to the eventual destination
server.
POP Server
A POP server enables
e-mail recipients to download received messages to their
own computers. Once the messages are retrieved by recipients,
the messages cannot be "put back" or stored
on the server.
IMAP Server
An IMAP server enables
users to retrieve mail and store mail (unlike a POP
server). Users can shuffle messages to and from the
IMAP server because both the mail directories and messages
are stored directly on the server. The IMAP protocol
is especially useful for people who check their e-mail
from multiple computers.
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