Symptoms

When you scan an Exchange On-Premises organization you see the following error

Error when 'Reading add-ins'. Error executing the command 'Get-App'. The specified mailbox "mailbox name" doesn't exist. Reason: mailbox name isn't a mailbox user.


Cause

The account performing the scan must be associated with an Exchange mailbox in order to execute the Get-App cmdlet.


Resolution

Ensure that the service account (or custom credentials if in use), has an associated Exchange mailbox.


More Information

The error can be replicated within the Exchange Admin Center.