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Chapter 9: Securing Internet Email
than processing mail out of one big queue as Sendmail does, Postfix uses four differ-
ent queues:
Maildrop queue
Mail that is submitted locally on the system is accepted in the maildrop queue.
Here the mail is checked for proper formatting (and fixed if necessary) before
being handed to the incoming queue.
Incoming queue
Mail initially received both from local processes via the maildrop queue and
from external hosts via Postfix’s smtpd process is preformatted if necessary and
then sent to the incoming queue. Here it will stay until there’s room in the active
queue.
Active queue
Since the active queue contains messages that Postfix is actively trying to deliver,
it has the greatest risk of something going wrong. Accordingly, the active queue
is intentionally kept small, and it accepts messages only if there is space for
them.
Deferred queue
Email that cannot be delivered is placed in the deferred queue. This prevents the
system from continuously trying to deliver email and keeps the active queue as
short as possible to give newer messages priority. This also enhances stability. If
your MTA cannot reach a given domain, all the email for that domain is assigned
a wait time and placed in the deferred queue so that those messages will not
needlessly monopolize ...