If you have purchased a mailbox (or multiple mailboxes) which you later had no use for, you can release the unused mailbox and receive a prorated refund for the remaining time on the mailbox subscription.
To release an unused mailbox go to the domain of the email and click on the Email tab.
Next to the button “Buy mailboxes” you will see a button “Release mailboxes.” Click on this box to start the release process. On the following screen, choose the number of boxes of each type to release and click on “Release.” If the + is greyed out, there are no more mailboxes of this type eligible for release.
The refund will be credited to your prepaid account.