Delete Mail Queue
If you are trying to delete your mail queue in Postfix for ISPConfig 3, try the flush command first before deleting all the mail in the queue. The flush command option will attempt to deliver every message in the deferred mail queue.
To list all mail in the queue:
postqueue -p
To flush the mail in the queue:
postfix flush
If the flush command option does not work, using the delete command will remove all the mail in the queue.
postsuper -d ALL
Bottleneck Analysis of Postfix
There is a reason that your mail queue got backed up. You should analyze and fix the problem. Deleting the queue does nothing but put a bandage on the problem, temporarily. Use the Postfix Bottleneck Analysis page (link in the Resources section below) to find a solution to your mail queue backup troubles.
Reserved.