Main.FlushThePostfixMailQueue
Flushing the Postfix mail queue
This command...
# postfix flush
...will flush the queue, but only if postfix is currently running. From experience, this may not work in time if a large unplanned deferred send in in progress ;)
You can also use postfix -f
While postfix is down, force-emptying the entire deferred queue can be done like so:
# postsuper -d ALL deferred
while ALL mail can be flushed with the following:
# postsuper -d ALL