An e-mail protocol is a set of commands and responses exchanged between a mail client and a mail server. The mail client is the program used to compose, send, and receive e-mail. Mail clients include Calypso, E-Mail Connection, Eudora, Lotus Notes, Microsoft Outlook and Outlook Express, Netscape Mail, and the Unix staples elm and pine.
Most users know their mail clients as well as or better than they know any other software.
What goes on during that communication depends on the chosen protocol. POP has become one of the Internet's more successful acronyms, with users and administrators referring to POP accounts, POP servers, and POP support. But POP's dominance for GUI operating systems is in danger of being overtaken by a relatively new kid on the block, IMAP, now in Version 4, Revision 1. At present, not all servers can handle the IMAP4rev1 proposal. An IMAP client has a choice of downloading only message headers, message headers and bodies but not attachments, or all message components.