Add Support For Microsoft Exchange Server for Service Desk

ZD Ticket: https://gitlab.zendesk.com/agent/tickets/92318

Right now, Exchange does not support Sub-addressing so Service Desk won't work:

https://social.technet.microsoft.com/Forums/office/en-US/6105396e-afd8-408e-a83d-eb9c20924da3/subaddressing-or-plus-addressing?forum=outlook https://serverfault.com/questions/201283/can-you-configure-ms-exchange-to-support-the-gmail-keyword-trick

This issue is to track demand for service desk to support this mail server.