After you click the Test & Enable Mailbox button, one or more of the test results shows as Failure.
One of the tests necessary to enable the mailbox for Server-Side Sync did not complete successfully.
1. Open the mailbox record.
Note: Mailbox records can be found by navigating to Settings, Email Configuration, Mailboxes, and changing the view to Active Mailboxes.
2. Review any notifications that appear in the yellow notification banner at the top of the form.
Note: If a message appears indicating the mailbox will not be processed until it is approved by an Office 365 administrator, this indicates a user with the Office 365 Global Administrator role needs to click the Approve button. More information
3. Click the Alerts section on the left side of the mailbox form.
4. Look for any errors and review the message that is displayed.
If no errors appear, navigate to Settings, Email Configuration, and then Email Configuration Settings. In the Configure alerts section, verify the Error option is selected. If this was not already selected, enable this option and click OK. Then click the Test & Enable Mailbox button again and check for any errors.
IMPORTANT: When performing the Test & Enable Mailbox action, make sure to select the checkbox that appears in the dialog.
5. Click the Learn more link if it appears. If there is an article designed to help with that specific error, this link will direct you to it.