To delete all the mail in the outgoing mail queue with Postfix simply do a:
postsuper -d ALL
To setup the retry delays, and queue lifetime add the following lines to main.cf.
#Max time between attempts to deliver a deferred message maximal_backoff_time = 30m #Min time between attempts to deliver a deferred message minimal_backoff_time = 15s # the maximal time a message is queued before it is sent back as undeliverable. maximal_queue_lifetime = 2d # the time between deferred queue scans by the queue manager # queue_run_delay = 1000s