I have a Front End OWA server that is getting multiple errors in the Event Logs under application.
The Errors are as follows:
Unexpected Exchange mailbox Server Error: Server [server.domain.dom] User: [firstname.lastname@example.org] HTTP Status code:  Verify that the Exchange mailbox Server is working correctly.
I haven't narrowed it down to 1 user either it seems to be affecting all users but just randomly. also the HTTP Status code will change from 500 to these:
Sometimes on my iPhone 5 when I go to look at my mail it just sits there and says Connecting but never finishes or takes 5 to 10min, sometimes the email will come into the phone and when I click on it to view it, it just sits there acting like its going to load more of the message but then says: Message hasn't been downloaded from the server yet.
It seems to work pretty well resetting the IIS or giving the server a reboot but I would like to know whats causing this error and why its now starting to effect everyone, I see this error in the event logs has been happening for a few months now but I haven't heard from any users about slowness or not getting mail on there devices up until now and now with myself experencing the issue its starting to make me wonder.