But there is still one missing piece to the puzzle! How does a recipient like Bob, running a user agent on his local PC, obtain his messages, which are sitting in a mail server within Bob’s ISP? Note that Bob’s user agent can’t use SMTP to obtain the messages because obtaining the messages is a pull operation, whereas SMTP is push protocol. The puzzle is completed by introducing a special mail access protocol that transfers messages from Bob’s mail server to his local PC. There are currently a number of popular mail access protocols, including Post Office Protocol—Version 3 (POP3), Internet Mail Access Protocol (IMAP), and HTTP.