550s in the mailq

Hello,

On a box that handles several mailing lists, I noticed that mail delivery was
being significantly delayed (to destination users, not local users). Upon
further inspection, I saw that there was a -ton- of stuff in the mail queue.

The output of the mailq program shows several items like this:

F12481E1913 2855 Tue Jun 15 16:15:17 csc-admin@lists.domain.com
(host mx01.1and1.com[217.160.230.11] said: 550 <projection@test1.com> quot
a exceeded)
projection@test1.com
(host ns.grove.net[205.244.12.2] said: 550 <tonytrout@test2.com>... User unkn
own)
tonytrout@test2.com
(host mxc.earthlink.net[207.217.125.27] said: 554 This mailbox is full. Please
try again later. for scottrm@test3.com)
scottrm@test3.com
(host mx01.lvcm.net[24.234.0.71] said: 550 <veoxi@test4.com>: User unknown in
local recipient table)
veoxi@test3.com
(host mx3.hotmail.com[64.4.50.239] said: 550 Requested action not taken: mailbox
unavailable)
zaak@hotmail.com

My question: since 550 errors are fatal, shouldn't that have generated a
bounce? Why is it in the queue?

Also, are there any updated FAQs on tuning Postfix for mailing lists? This
happened with 1.1, but I'd like to upgrade to 2.1, and want to be sure I don't
hit any limits again.

Bob