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 /etc/aliases). Delivered 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 in the mydestination parameter.
Delivers mail to other systems, usually on the same network. Relay domains are generally configured on gateway systems when Postfix accepts mail for an entire network. The gateway system relays messages to the correct internal mail system. Deliveries are handled by the relay transport, which is simply a clone of the smtp agent, but it is optimized for making deliveries to internal systems on a local network. Lists relay domains in the relay_domains parameter. Mail relaying is discussed in Chapter 9.
Delivers mail for virtual mailbox domains. Virtual mailbox domains are used for hosting multiple domains using a separate mail spool that contains mailboxes for many separate domains. Email users typically do not have system accounts on the mail server. Lists virtual mailbox domains in the virtual_mailbox_domains parameter. Virtual hosting is discussed in Chapter 8.
Deliveries to nonlocal domains are handled by the smtp transport. It determines where to deliver messages for any nonlocal domain through DNS lookups. Virtual alias addresses are resubmitted to Postfix for delivery to the new address, at which point they'll be handled by one of the above transports.
The rest of the chapter discusses the details of local delivery.
General Configuration and Administration
Email and DNS
Local Delivery and POP/IMAP
Hosting Multiple Domains
Blocking Unsolicited Bulk Email
Transport Layer Security
Appendix A. Configuration Parameters
Appendix B. Postfix Commands
Appendix C. Compiling and Installing Postfix
Appendix D. Frequently Asked Questions