Postfix queue

Count queued messages:
# postqueue - p | grep "Requests"

To flush the mail queue:
# postfix flush
OR
# postfix -f

To see mail queue, enter:
# mailq

To remove all mail from the queue, enter:
# postsuper -d ALL

To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred

Reading message content on postfix queue:
# postcat -q MESSAGE_ID

Postfix log:
/var/log/maillog