Messages hanging around in queue for awhile (in multi-site Zimbra install)

[Service]

Good morning all,
I recieved a complaint that sending/recieving messages sometimes takes hours, and these messages are to another site within our Zimbra installation and some to external recips. I've made it a practice to watch the queue with Zimbra Admin and I do notice some messages get a little sticky in there that seem like they shouldn't, and I'll just requeue them and they go aways. Obviously not the best solution to a problem like this. Am using the ZCS edition of Zimbra, if that matters. Another thing which I doubt is relevant is that all users currently on the system are using it as a POP3/SMTP with Outlook/OutlookExpress/etc as client, so can likely rule out a Tomcat issue/misconfiguration (right?).
On a couple searches I found I can setup a cron job to flush the queue at specified intervals with

/opt/zimbra/bin/postfix stop/start

postqueue -f
but I wondered why the system is exhibiting this behaviour and how to approach it best. Thanks for any guidance on this.

[17629anand]

what is in /var/log/zimbra.log for the queue files that are stuck?