As far as I can see everything is the same (certainly CentOS, BIND & fetchmail are the same) but the results would seem to indicate otherwise.
I don't want to waste your time on a trivial problem so I'll dig around some more, I only asked just in case it was a stupid error on my part. Thanks for the info on fetchmail I'll see what i can do with that.
Thanks also for that info on the mail queue, it's now been removed.
Do you plan to have any queue management features for deleting this kind of rubbish in Zimbra in the longer term?