Exchange mailboxes quota has been reached

If you receive the control panel error:

"Exchange mailboxes quota has been reached"

To add additional mailboxes:
Click on My Ecommerce, My Services
Click on Upgrade Service
Select Exchange Hosting Additional Mailboxes, Select Monthly, Select Offline Payment and Continue
Check the box at the bottom that agrees to Terms and Conditions and Select Complete Checkout.

There will now be space to create the mailbox.