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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 current standard.
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 Gmail, Outlook.com and Yahoo! Mail also
providing support for either IMAP or POP3 to allow mail to
be downloaded.
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. In the late 90's and
early 2000's, fewer Internet Service Providers (ISPs)
supported IMAP due to the storage space that was required on
the ISP's hardware. Contemporary e-mail clients supported
POP, then over time popular mail client software added IMAP
support.
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. |