Understanding POP and IMAP

The POP and IMAP protocols are designed to provide access to stored mail using a relatively lightweight, text-based protocol. They have different goals, architectures, and feature sets, and their implementations vary somewhat in their capability and adherence to their defining standards. However, they’re increasingly prevalent, particularly on devices like the Kyocera Smartphone and other handheld and mobile devices that typically don’t have enough screen real estate to deal well with Outlook Web Access.

POP’s original genesis came in the early 1980s after some bright folks realized that not every computer on a network had the resources necessary to act as a mail server, and some machines would be better off with a client-oriented protocol. Accordingly, POP was designed around the model familiar to users of old-style UNIX mailers: the client connects to a server, downloads new messages, and stores them locally on the client. POP doesn’t support folders, and it has very limited support for client-side caching and partial message downloads, both of which are critical for mobile users. The original POP definition also had the significant limitation of supporting only plaintext authentication, although more modern POP clients support a variety of more secure authentication methods. The biggest drawback of POP for most users is that it was designed to work with one account on one computer. POP users often get confused (or angry) because the messages they need have been downloaded to their home computer while they’re at work (or vice versa) and then deleted from the server.

IMAP was designed to be a more general and flexible protocol; in particular, IMAP explicitly supported the notion of arranging mail in folders (either manually or by rules) and leaving all messages on the server while maintaining a local cache of selected messages. IMAP works well with multiple computers and accounts because each client can maintain its own message cache without affecting the others. Additionally, the basic IMAP protocol specification has been extended to specify support for several additional authentication mechanisms.

Both IMAP and POP are widely supported. Microsoft Outlook (Windows only) and Microsoft Outlook Express (Windows, Solaris, and MacOS) packages support both protocols, as do Netscape Communicator, Qualcomm’s Eudora, Microsoft Entourage X for Mac, and a host of lesser known mail clients like Pegasus Mail, PowerMail, and The Bat! In general, Exchange follows Jon Postel’s admonition, “Be strict when sending and tolerant when receiving,” so there are few interoperability problems with well-written clients.