Some commands that aid with managing the mail queue in Postfix.

Show mail queue size:

mailq

or

postqueue -p

Show queue domain and age:

qshape

qshape deferred

-s Display the sender domain distribution instead of the recipient domain distribution

Queues:

deferred

recipients delivery failed for a transient reason (it might succeed later), the message is placed in the "deferred" queue

active

Messages in the "active" queue are ready to be sent (runnable), but are not necessarily in the process of being sent (running)

incoming

All new mail entering the Postfix queue is written into the "incoming" queue

hold

Administrator defined rules place messages into the "hold" queue and stay there until the administrator intervenes

maildrop

Messages that have been submitted via the sendmail command, but not yet brought into the main queue, await processing in the "maildrop" queue

Purge a single email:

postsuper -d [message id]

Purge all emails:

postsuper -ALL