Mac Outlook 15.11 2015 "connection to the server failed or was dropped" (Error # -3253)
I had this problem, too, and about the same time Safari started acting up by not loading pages. I found that the problem was the Avast Anti-Virus Software that I had installed on my machine. Once it was removed, the problem was solved: no more Safari issues and no more Outlook issues, specifically Error -3253.