Email was first introduced long before personal computers became popular, and most email was read from the same central computer on which it was stored. With the advent of the Internet and the widespread use of personal computers, it became much more common for email to be fetched from a central server and read by a client on a personal computer. As SMTP is a sending-based protocol, new communication protocols were devised for fetching mail from the central servers. The most popular are known as the Post Office Protocol 3 (POP3) and the Internet Mail Access Protocol (IMAP).

POP3 is a very simple protocol with limited capabilities. With it, a client can request messages, delete messages, and detect new messages. However, that is essentially the extent of its feature set. Most POP3 mail clients use POP3 to retrieve the messages and store them locally, then either delete them from the central server or leave them there as a backup copy. Because the protocol is so...