Postfix is structured as a series of small programs that serve limited roles.
Stage 1, the pickup stage, is made up of two programs, pickup which reads mail from the maildrop file and smtpd which reads mail from the internet.
Stage 2, the cleanup stage, rewrites the email to a standard form, handles virtual addresses and bounced mail.
Stage 3, the queue stage, handles a queue of incoming mail and places it into the active or, if there is a problem, the deferred queue.
Stage 4, the delivery stage, resolves addresses and either
places the mail in a local mailbox or sends it out to another node
on the internet.