Building Webmail Over IMAP Can Be a Good Idea. An open standards approach to providing an email service will usually offer three access methods: POP (Post Office Protocol), IMAP (Internet Message Access Protocol), and Web access. POP and IMAP server implementations will generally access a message database into which messages are delivered. Webmail can either access the message database directly, or indirectly through POP or IMAP. Because POP is primarily for message delivery, it is an inefficient choice. IMAP is designed for message access and can be used as an efficient access mechanism to support user queries. This has a number of advantages.

For a user with multiple mailboxes on several servers, a fully decoupled interface permits a single Web user interface to be used for all the mailboxes. Although not widely offered today, such a service would be attractive to some users.

For a service provider, this use of IMAP would enable webmail to fit efficiently into a multitier Web deployment. The IMAP servers can sit at the back end and provide the core message access services. Webmail can be located in a middle tier in front of IMAP, which gives good horizontal scaling and security. Additional use of IMAP proxies will add to the flexibility and scalability of the architecture -- users can connect to any Web server in a clustered pool and be transparently connected to their "home" IMAP service.

This approach is commonly used by both open source and commercial webmail interfaces, but it is not universal. This architectural choice is one consideration in selecting a product or service. One major example of an IMAP open source webmail user interface is SquirrelMail. ... Steve Kille