After OX mail has been enabled on your account, it is available to all email addresses.
However, when creating a new email address in the Email section, by default it only enables altmail access.
This can be corrected in the Users section of your control panel.
Click on the Display Name of the new email account, and then click on Add Service.
Ensure that both Email and Blacknight App Suite are ticked and then click Next.
There are several more screens that nothing needs to be changed on, so just keep clicking on Next until Finish.
Alternatively, you can create all new email accounts from the Users section, and give the Email and Blacknight App Suite services at the time of creation.
OX Mail accounts can be logged into at https://ox.blacknight.com
If you need to migrate existing contacts from altmail to OX Mail, please follow the steps in this guide: https://help.blacknight.com/hc/en-us/articles/212519529-How-to-Export-Contacts-from-Altmail-and-Import-into-OpenXchange-OX-Webmail