We had similar issues in the past, but not at the frequency in which you are experiencing it. Ours would happen once every 2 months or so. When it happened in our case, all of the reported users were always on one particular Exchange server. Restarting the BES also resolved it short term, but since all of the reported (and most didn't report it) were repeatedly on the same Exchange server, we were able to determine the issue was with Exchange.
I do recall it was fairly simple to determine the cause after examining the log files. I just wish I could recall what the actual issue was. I'll see if any of my colleagues can recall it.
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