I have a Blackberry 7250 and three times in the last month all of the emails have suddenly disappeared - it looks like each one is being quickly deleted. The last time, it deleted everything except the most recent 25 emails.
Originally, I was told it was because the Blackberry ran out of memory, but I have been deleting contacts etc.. and had 10MB out of 32 MB available. There was a very similar question on Experts-Exchange http://www.experts-exchange.com/Programming/Wireless_Programming/Blackberry/Q_21691073.html
but it appeared to be a memory issue, and this one has memory available.
The unit is set to keep emails forever.
We use a BES 4.0 with all the service packs and hotfixes. The BB is set to delete on both the BB and server but luckily, when the emails vanished, they were still in Outlook, though no longer accessible from the BB.