POP, or Post Office Protocol, is an Internet standard protocol employed by a local email client, like Outlook or Eudora, to retrieve email messages over a TCP/IP connection to a remote server. POP is one of the two most commonly used Internet standard protocols for email retrieval, with the other being the Internet Message Access Protocol or IMAP protocol. Most webmail services, like Yahoo mail and Gmail, provide both POP and IMAP services.
While most POP clients provide the option of leaving email on the server after it has been downloaded, POP clients typically connect to the server, download all the messages, save them to the local user’s device as new messages, remove the messages from the web server, and then disconnect from the server.
Originally developed in 1984, POP2 was standardized in 1985 and the most recent iteration of the standard is POP3. Both POP2 and POP3 are currently in use, with POP2 servers listening at well-known port 109 and POP3 servers at well-known port 110. In POP3, encryption is requested after the protocol is initiated, by use of STARTTLS, or POP3S, which uses either Secure Sockets Layer (SSL) or Transport Layer Security (TLS) to connect to the server via TCP port 995.