Question / Problem:
When setting up Import Source for EWS and using Office 365 exchange web service, the connection test is successful, but when we try to complete the import setting it throws an error: "The entered mailbox is already in use with the specified URL. Please enter a unique mailbox to poll."
Answer / Solution:
This error often occurs if you are using a shared mailbox EWS.
This is a security check that was implemented to make sure that you don't enter the same mailbox twice.
This security check might have to be improved in order to accommodate for the "Shared Mail Box"
You can work around this security check by making a small change to the Username.
Current Username value: firstname.lastname@example.org
Change it to the value. : Myname@mydomain.com
For the new EWS mailbox connection.
So turn one of the lower case characters of the username field to an Upper case character (or vice versa).