Connecting to Exchange 2013 EWS with Exchange 2010 Folder Permissions

Scenario: When making Exchange 2013 EWS calls that proxies down to a Exchange 2010 mailbox connecting via shared folder permissions of another Exchange 2010 mailbox, the EWS connection may be unstable. Programs such as Starfish, SoapUI, or other programs making EWS calls, may or may not connect consistently and some errors encountered are 400 Bad Request errors.

Resolution: We created a new mailbox in Exchange 2013 and gave this mailbox the folder share permissions to the Exchange 2010 mailboxes. Making EWS calls to Exchange 2013 by using the 2013 mailbox to connect to the mailbox folders of the 2010 mailbox, EWS had become stable and successful.