To flush or purge the postfix mail queue, just enter this command

postfix -f

But if you need to delete an individual email from the queue, you'll first need to see the queue. Traditionally you use mailq this time we'll use:

postqueue -p

And the output should show all messages in queue:

5642B4D8647* 1683500 Tue Jun  3 08:37:27  [email protected]
                                         [email protected]

9359B4D82B1* 1635730 Tue Jun  3 08:36:53  [email protected]
                                         [email protected]

The first number is the message ID, if you only want to delete one of them, enter:

postsuper -d 5642B4D8647

That will only delete one email for the queue, that specific email you want to delete from it.

If you want to delete all deferred mails, you can use:

postsuper -d deferred