Here is the few useful commands for POSTFIX which may help you while working with POSTFIX.
* List mails present in the queue: postqueue -p or mailq
* TO flush the mail queue: postfix -f
* To remove all mail from the queue: postsuper -d ALL
* To remove all mails in the deferred queue: postsuper -d ALL deferred
* Deliver Queued Mail for a Domain: postqueue -s thedomain.com