DHCP&_160;· DNS&_160;· FTP&_160;· Gopher&_160;· HTTP&_160;· IMAP4&_160;· IRC&_160;· NNTP&_160;· XMPP&_160;· POP3&_160;· RTP&_160;· SIP&_160;· SMTP&_160;· SNMP&_160;· SSH&_160;· TELNET&_160;· RPC&_160;· RTCP&_160;· RTSP&_160;· TLS (and SSL)&_160;· SDP&_160;· SOAP&_160;· GTP&_160;· STUN&_160;· NTP&_160;· BGP&_160;· RIP&_160;· (more)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. POP3 and IMAP4 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.
POP3 has made earlier versions of the protocol, informally called POP1 and POP2, obsolete. In contemporary usage, the less precise term POP almost always means POP3 in the context of e-mail protocols.
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.
