pfqueue is an effort to give postqueue/mailq/postsuper/exim4 a console
(ncurses) interface: it won't add any particular functionality to those
provided with MTAs themselves, but will hopefully make them easier to
use.

It's a real-time queue scanner, that shows per-queue lists of existing
messages; the messages can be shown, deleted, put on hold, released or
requeued.

Just for example, it may be useful to inspect a traffic jam at a given
time, to see what is falling into and unexpectedly crowding your deferred
queue.