I have a AD network at functional level 2008 and (2) DC's currently running Windows Server 2012 R2. Prior to promoting those servers, my users were getting randomly locked out of AD, so I made the transition to the new domain controllers. I have not raised the function level as of yet to 2012. I did implement many of my users with new email hosting (some exchange and some POP/IMAP), it seems to have started around the same time.
My hosted exchange is not integrated into my AD.