POP3 (Post Office Protocol)
In computing, the Post Office Protocol (POP) is an application-layer
Internet standard protocol used by local e-mail clients to retrieve e-
mail from a remote server over a TCP/IP connection. POP has been
developed through several versions, with version 3 (POP3) being the
Virtually all modern e-mail clients and servers support POP3, and it
along with IMAP (Internet Message Access Protocol) are the two most
prevalent Internet standard protocols for e-mail retrieval, with many
webmail service providers such as Google Mail, Microsoft Mail and Yahoo!
Mail also providing support for either IMAP or POP3 to allow mail to be
POP supports simple download-and-delete requirements for access to
remote mailboxes (termed maildrop in the POP RFC's). Although most POP
clients have an option to leave mail on server after download, e-mail
clients using POP generally connect, retrieve all messages, store them
on the user's PC as new messages, delete them from the server, and then
disconnect. Other protocols, notably IMAP, (Internet Message Access
Protocol) provide more complete and complex remote access to typical
mailbox operations. Many e-mail clients support POP as well as IMAP to
retrieve messages; however, fewer Internet Service Providers (ISPs)
A POP3 server listens on well-known port 110. Encrypted communication
for POP3 is either requested after protocol initiation, using the STLS
command, if supported, or by POP3S, which connects to the server using
Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-
known TCP port 995.
Available messages to the client are fixed when a POP session opens the
maildrop, and are identified by message-number local to that session or,
optionally, by a unique identifier assigned to the message by the POP
server. This unique identifier is permanent and unique to the maildrop
and allows a client to access the same message in different POP
sessions. Mail is retrieved and marked for deletion by message-number.
When the client exits the session, the mail marked for deletion is
removed from the maildrop.
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