Speaking in a generic way, Postfix can handle two kinds of mail: inbound mail and outbound mail. When Postfix receives mail messages, it can be mail from a local user to another local user, and it can be e-mail that is received over the network for some local user. Also, Postfix is responsible for handling outbound mail. This outbound mail can be delivered to a local user (when the sender and recipient are on the same server) or to an e-mail user on a remote server, but it can also be an undeliverable mail.
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