POP3 server authentication
POP stands for Post Office Protocol. This is used to describe how e-mail clients
interact with mail servers. The POP3 Server is a type of mail server used for incoming
mail. In simple terms, POP servers provide a mail-drop service (a temporary mailbox to
leave messages so they can be picked up at the recipient's convenience.) When users
connect to their ISP POP servers, their e-mail software interface with the server and
download any messages for them. POP is only used to receive messages, it is not used to
send them. In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an
application-layer Internet standard protocol, to retrieve e-mail from a remote server
over a TCP/IP connection. Many subscribers to individual Internet service provider e-
mail accounts access their e-mail with client software that uses POP3. The design of POP3 and its procedures supports end-users with intermittent connections
(such as dial-up connections), allowing these users to retrieve e-mail when connected
and then to view and manipulate the retrieved messages without needing to stay
connected. Although most clients have an option to leave mail on server, e-mail clients
using POP3 generally connect, retrieve all messages, store them on the user's PC as new
messages, delete them from the server, and then disconnect. In contrast, the newer, more
capable Internet Message Access Protocol (IMAP) supports both connected (online) and
disconnected (offline) modes of operation. E-mail clients using IMAP generally leave
messages on the server until the user explicitly deletes them. This and other aspects of
IMAP operation allow multiple clients to access the same mailbox. Most e-mail clients
support either POP3 or IMAP to retrieve messages; however, fewer Internet Service
Providers (ISPs) support IMAP. The fundamental difference between POP3 and IMAP4 is that
POP3 offers access to a mail drop; the mail exists on the server until it is collected
by the client. Even if the client leaves some or all messages on the server, the
client's message store is considered authoritative. In contrast, IMAP4 offers access to
the mail store; the client may store local copies of the messages, but these are
considered to be a temporary cache; the server's store is authoritative.
POP3 works over a TCP/IP connection using TCP on network port 110. E-mail clients can
encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection is negotiated using the
STLS command. Some clients and servers, like Google Gmail, instead use the deprecated
alternate-port method, which uses TCP port 995.
