Figure 1-2 illustrates a simple example of message transmission where Postfix handles the responsibilities of the MTA and local delivery. As the MTA, Postfix receives and delivers email messages over the network via the SMTP protocol. For local delivery, the Postfix local delivery agent can deposit messages directly to a message store or hand off a message to a specialized mail delivery agent.
This example shows Postfix as the SMTP server at both ends of the email transaction; however, since Postfix is based on Internet standards, the other email server in this example could easily be any other standards-compliant server. Postfix can communicate with any other server that speaks SMTP (and even some that are not quite fluent). In our example, Heloise wants to send a message to Abelard from her address (firstname.lastname@example.org) to his address ( email@example.com.) Heloise uses her email client to compose her message, which passes it to her MTA (using SMTP). As it happens, her MTA is a Postfix server that allows her to relay messages. After accepting the message from Heloise's email client, the Postfix server determines where Heloise's message needs to go, based on Abelard's email address. Using DNS (see Chapter 6 for more information on DNS and email) it figures out which SMTP server should accept messages for Abelard's domain (postfix.org) and contacts that server (using SMTP). Abelard's Postfix server accepts the message and stores it until Abelard is ready to pick it up. At this point Postfix's job is done. When Abelard is ready to retrieve his messages, his email client, using POP or IMAP, picks up Heloise's message.
This example leaves out the details of the complicated tasks involved when Postfix delivers mail. In the case of messages with multiple recipients, Postfix has to figure out where to deliver copies for each recipient. In case one or more recipients cannot receive mail due to a networking or systems problem, Postfix has to queue the message and retry delivery periodically. From a user's point of view, the Postfix piece of the operation is nearly invisible. From the Internet mail system's point of view, Postfix handles most aspects of email message delivery.