The very latest version of Outlook (some time within the past week) has removed the ability to use third party connectors. That means users of Zimbra, Kerio, and other Exchange alternatives that rely on a third party connector to function are going to be in for a very nasty shock when they are no longer able to set up new installations. It seems as if existing users are still okay for now, but that may change.
This change has apparently caught everyone by surprise. Zimbra support is still triaging the issue, and Microsoft support people are not in a position to help since this appears to be an intentional change rather than a bug.