A customer cannot create an Exchange mailbox or distribution list in POA Control Panel, the corresponding background task fails with an error message like that below:
The proxy address "SMTP:firstname.lastname@example.org" is already being used by "hosting.local/Microsoft Exchange System Objects/sales". Please choose another proxy address.
There is no other Exchange mailbox or distribution list with the same email address in the customer's subscription in POA.
Most likely, the customer created another Exchange object (e.g., a mail-enabled Public Folder) with a conflicting email address directly on the Exchange server using Outlook or OWA. So, POA knows nothing about the existing email address and allows the customer to create a mailbox or distribution list with the same email address in the Customer Control Panel. Finally, the background task 'Create mailbox with E-Mail Address' or 'Create distribution list' fails with the error message about a conflicting email address.
One may use the ADSIEDIT.msc tool on the Active Directory Domain Controller to confirm that the Exchange object has a conflicting proxy address; the full path to the AD object is given in the error message in the failed task in POA Task Manager:
Resolve the conflict: consider either converting the conflicting Public Folder to the mail-disabled one, or changing its email address (and proxy address as well) to another one that does not conflict with the email address of the mailbox you want to create. Then, re-run the failed task in POA.