I have a few mailboxes I created way back in Exchange 2003 that are shared between several people, so after migrating to Exchange 2013 I decided to reconfigure those shared mailboxes “correctly” as actual “Shared” mailboxes. That was easy.
Then I discovered a small problem. Shared mailboxes only work when using the full Outlook client or through OWA. Up to now, Activesync was the primary method of accessing these mailboxes, and you can’t access a Shared mailbox using Activesync!
The reason is pretty simple. Shared mailboxes are created based on a disabled Active Directory user account. This is great for security, and also great because you don’t need a license for that mailbox. By disabling the AD account however, you have no way to authenticate using Activesync.