We migrated from Exchange 2007 to 2013 a little under a month ago.. Ever since one user has been complaining that several of his emails have been ending up in the Outlook Junk Mail folder. All of the messages are tagged with: "This message was marked as spam using a junk filter other than the Outlook Junk E-Mail filter."
Problem is, I can't find anything that could possibly be filtering these messages. We use a Barracuda spam appliance, and the messages clear it with no problem (in fact most of them are whitelisted - the most notable offender is emails from our phone provider containing voicemail messages - 100% of these are filtered in testing). I've never installed the Anti-Spam agents on the exchange server, and a quick glance via powershell indicates they aren't enabled.
I've verified that the filtering is happening outside of his PC. The messages show up in Junk Mail on his smartphone immediately even with his PC disconnected.
Any other places I can look to try to figure this out? Any obvious things I might be overlooking?