We've got a user that's having random sent messages disappear after sending, they aren't stuck in the Outbox and they never reach the recipient. This used to occur on our Exchange 2007 server but we've since moved to Exchange Online; the problem remains. This only happens with Cached Mode enabled, disabling is too slow with O365.
We use Webroot for AV, but they haven't been able to find an issue with their software. The user is on Outlook 2013 and it's up-to-date.