Chapter 1 explained that POP and IMAP are protocols that deal with how users retrieve their email messages from message stores. Postfix is a mail transfer agent and does not implement POP or IMAP. This chapter looks at how Postfix delivers messages and how they are read by POP/IMAP servers. There are many POP/IMAP servers available, and the information presented here should be applicable to any standards-conforming server. The last part of this chapter deals with configuring Postfix to work with the Cyrus IMAP server. Before we look at local delivery, we'll first discuss more broadly the different delivery transports Postfix uses. Transports other than local are discussed in subsequent chapters.
Postfix offers delivery for four different classes of recipient addresses: local, relay, virtual alias, and virtual mailbox. How you configure the domains you accept mail for determines the delivery method used by Postfix. The following are the delivery transports used by Postfix:
Delivers mail on the local system. Each address has an account on the
system or comes from the local aliases file (historically
messages go to the system's mail spool or mail files in individual
home directories. Deliveries are handled by the local delivery
agent or passed to a custom delivery program. Lists local domains
Delivers mail to other systems, usually on the same network. ...