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