OX mail is available to be added to all email addresses.
New email addresses you create in the Email section, by default have 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.
Click Next past the screen with a tickbox for Mobile.
On the next screen, with the contact information, blank out the Given Name and Surname, then click 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