Messaging application protocols

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Just as the process of writing a letter and putting it in an envelope is a different and necessary function from the transfer and delivery function of a postal service, electronic mail needs user application programs to create and read mail, but also protocols to transfer and deliver electronic envelopes.

There are two basic roles in electronic mail transfer: client to mail server, and mail server to mail server. None of these include the human interface, which is in the user application program that runs on the user's client computer.

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

Simple Mail Transfer Protocol (SMTP) is fairly universal as the protocol between mail servers. SMTP, while not a full peer-to-peer protocol in the sense that all potential peers are treated equally and openly, is more symmetrical than most client-server protocols. Either end can initiate SMTP protocol exchanges.

Finding mail servers

  • MX record
  • Relays?