Most email is transmitted via one of just a few standard protocols. Simple Message Transfer Protocol (SMTP) is the protocol generally used for sending email messages, whereas POP3 and IMAP are used for receving email messages. POP (Post Office Protocol) has been largely replaced by IMAP (Internet Messaage Access Protocol). Systems using POP will, under default settings, download email to a user's computer and delete it from the server, meaning there could be no back-up if a PC fails. IMAP email will be retained on a server, as only the headings for the email are downloaded to the user account, until the user actually opens an email message. MAPI (Message Application Programming Interface) is the protocol that Outlook uses to communicate with the Microsoft Exchange server. MAPI email is stored on a server, and won't necessarily show up on a user's PC.