Every couple weeks, one user or another in my office will report that Outlook for Mac 2011 is re-downloading hundreds or thousands of emails. Initially I thought these folks needed to set Outlook to delete mail from the server, but in the latest instance, the user definitely has that property set. The size of his mail file on the server is tiny, but this morning his Outlook started "downloading" nearly 5,000 messages back to at least 2012. He's using version 14.3.4, but I believe newer versions have seen this behavior as well.