thaxor

I started the morning with this - Critical Errors in the Event Logs: 150. At least they were all the same error:

Unexpected Exchange mailbox Server error: Server: [server.local] User: [user@domain.com] HTTP status code: [422]. Verify that the Exchange mailbox Server is working correctly.

So after ~1 hour of searching I find at least  5 solutions including deleteing/recreating virtual folders, editing \etc\hosts, editing registry settings, removing SSL encryption requirements, and tweaking some IIS settings.

Right before embarking on this huge list of fixes I read the last comment in this thread.

I have hunted high and low and finally grabbed the users phone and found that there were items in the outbox that could not be sent and there were a few old drafts. Looks like the outgoing mail that was stuck was the culprit.
So I asked if I could delete them and no errors in 30 minutes so far from that user.

I asked the user to delete everything in drafts/outbox and sure enought here was an email about 1 week old stuck in there. Here we are about 1 hour later and no errors. So if this behavior is only presenting on a few devices and not all users I suggest you ask them to delete anything stuck in the outbox/drafts folder.