When trying to display a message in the Postfix 'Mail
Queue' list the browser returns 'The message is no
longer in the queue' if that particular message is in
the 'HOLD' queue. Also attempts to delete messages in
the 'HOLD' queue do not get removed. I'm not sure if
this may be related to the fact that messages in the
HOLD queue appear to have a trailing exclamation mark
on their message ID.