Messaging application protocols
There are two basic functions in electronic mail: client to mail server, and mail server to mail server. Simple Mail Transfer Protocol (SMTP) is fairly universal as the protocol between mail servers. SMTP, while not a full P2P protocol in the sharing sense, is more symmetrical than most client-server protocols. Either end can initiate SMTP protocols exchanges.
None of these include the human interface, which is contain in the client application program (i.e., as distinct from any client protocols that communicate with other computers).
Client to server
Between the end user workstation and the mail server, however, there are two major protocols, both with secure variants. The Post Office Protocol, Version 3 (POP3) and the Internet Message Access Protocol (IMAP) both manage mail between the client workstation and the mail server. They differ in where they keep mail folders: POP3 assumes it is on the client while IMAP assumes it is on the server.
A third option is to use a web client that talks to the server and looks at messages there through an application. The advantage of web mail access is that it can come from any web browser that can gain access, but webmail applications tend to be less feature-rich than applications using POP3 or SMTP.
Server to server
Finding mail servers
- MX record