We had a flood of these messages Monday, and it'd tapered off to almost none today. I am suspecting a connection, given your observations, to some related problems we had then. In particular, the trust with one domain was broken so a number of BB accounts had problems because the BES and Exchange servers couldn't talk with each other.
We commonly have some users with huge numbers of queued messages for a variety of reasons. Most often it's units being transferred and the server accounts aren't updated, but other things too.
At least I did manage to get our vendors to sit up and try to help our server engineers on this. A senior RIM engineer said this was a known problem with an earlier version of BES software, but not with the latest version that we're running. We'll see what develops from that. I'll let you know.
Steve