Its funny - I was trying to find the same thing yesterday, as my secondary MX host was required for a few hours, and the queue had a number of messages in it and it was taking its time to retry sending the messages to the primary.
Anyway:
will get the server to try to resend all the messages in its queue.
On a related note - does anyone know how to set the time between retrys for sending messages in Postfix. IE: I would like it to try every 15 minutes to resend the messages in its queue, rather than waiting for an eternity to send one message at a time.
--Ian