Chapter 10: Serving Mailboxes

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Highlights

Simple mail transfer protocol (SMTP) transfers mail between servers and sends (injects) new messages, but it's not well suited to client systems that want to retrieve mail because it wasn't designed for that. Two protocols designed to allow clients to access and retrieve mail remotely are the Post Office Protocol (POP) and the Internet Mail Access Protocol (IMAP).

POP, the current version of which is POP3, was designed specifically for providing clients access to their mailboxes. Although it's possible to configure POP3 clients to store the user's mailbox on the server, it's usually used only to serve unread messages. In this case, the user's primary mailbox resides on the system that runs the Mail User Agent (MUA). This chapter uses POP and POP3 somewhat interchangeably because the earlier versions are obsolete and rarely encountered.

IMAP is a newer, more advanced-and more complex-protocol designed to provide remote access to a mailbox that resides on the server.

Both POP3 and IMAP are widely implemented in MUAs including Eudora, Netscape, Mutt, and Outlook Express.

Whether you choose to support POP3, IMAP, both, or neither depends on many factors, including:

• Do you want or need remote access to mailboxes?

• Centralized mailbox storage (IMAP) allows easy backups.

• Centralized mailbox storage requires more centralized resources and creates a potential single-point-of-failure.

• Centralized mailbox storage allows access to a mailbox from any client system.

• Centralized mailbox storage allows alternative access mechanisms such as Web mail (a Web-based MUA).

• MUAs must support the protocols deployed and vice versa.

qmail includes a POP server, qmail-pop3d, but it's not configured and activated as part of the qmail installation process. You can also use one of the other POP or IMAP servers available; however, some of them were written for Sendmail and require some reconfiguration to use with qmail.

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