Use another Exchange Mailbox for CRM Admin

Hi,

Is it possible to use the email of a User who has not been assigned a CRM license as the email for a CRM Admin User.

e.g. User: John Smith has an exchange license but isn't active in any other area of Office 365. The CRM Admin User who has a CRM license wants to use John Smiths email to send and recieve messages in CRM. Essentially setting CRM Admin User Mailbox to johnsmith...

I have tried this in a test environment with no success, tests always return "failure". I can also not add John Smiths email to the CRM Admin User within Office365 because it's already used by John Smith's User.