Ben,
The domain settings work together to control EWS/MAPI access for a domain. You need to set the toggle to the enabled position and specify an appropriate limit for the domain. Once EWS/MAPI is enabled and a limit is set, then you can use the toggle for a user to enable them to access EWS/MAPI.
Andrew Barker
Software Developer
SmarterTools Inc.
www.smartertools.com