This feature is intented to let the user (re)queue certain
Why only certain messages?
On a busy server used to "relay" messages to remote
domains the queue can be very big (10k of messages in
the queue) and sending qmail-send ALRM will
rescheduled the entire queue.
If one or more remote domains are "down" qmail waits for
timeout to these remote servers making the qmail-server
inresponsive for some time. This could be avoided if only
certain messages is reschedule.