After clicking Test & Enable Mailbox on a mailbox record in Dynamics 365, the Appointments, Contacts, and Tasks test result shows as Failure and the following message appears within the Alerts section:
"An internal Microsoft Dynamics 365 error occurred while synchronizing appointments, contacts, and tasks for the mailbox [Mailbox Name]. The mailbox didn't synchronize. The owner of the associated email server profile [Email Server Profile Name] has been notified.