Mail, Calendar and Contact REST APIs exist for connecting to Exchange (365 or 2016) and would be required for an organisation moving away from the currently supported email protocols.
Inbound - POP3, IMAP and MAPI
Outbound - SMTP and MAPI
Use Mail API to connect to Mail boxes for existing vFire Incoming and Outgoing Email functionality.
Enhance vFire to allow for Tasks and Requests to be synchronised with Exchange Calendars to publish Change Windows and the Forward Schedule of Change etc
EWS (Exchange Web Services) is also available, however Microsoft are moving toward and significantly investing more in the REST APIs than EWS so that soon all EWS functionality will be available through a single modern API.
Exchange Web Services is now supported, as is Office 365. Additional authentication methods including OAuth 2 was added in 10.3.
Ability to publish entries into Exchange calendars from a workflow would be useful