Under Postfix MTA,
just enter the following command to flush the mail queue:
# postfix flush
OR
# postfix -f
To see mail queue, enter:
# mailq
To list all mail of queue, use one of the following commands.
# postqueue -p
To remove all mail from the queue, enter:
# postsuper -d ALL
To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred
