Qmail, flushing the queue
First you need to identify the qmail-send process by pid:
server# ps ax | grep qmail-send 632 con- I 0:02.82 qmail-send
server# kill -ALRM 632
Qmail will now begin attempting to deliver anything in its queue, regardless of whether it's scheduled to retry it yet or not. NOTE: do not substitute the killall command; under FreeBSD (as of 4.0-6.1) the killall command does not appear to actually pass the ALRM signal; it just passes the default TERM signal and kills the process entirely. Not what you're looking for.