In order to be able to add mailboxes to all of your domains, you must add it to accepted domains first
How to add a domain
1. Open WorldPosta Control Panel (admin.worldposta.com).
2. in System section, click on Domains.
3. Click on Add Domains.
4. Click on Domain to add a Top-Level Domain (TLD) or Sub-domain to add a sub-domain for already added top-level domain
5. In the Domain Name field, enter your domain.
When you finish, click on Add Domain.
Now, in order to be able to add your domains to the domains list, you must add it to the accepted domains.
1. In the Hosted Organization section, click on Accepted Domains.
2. Click on Add New Domain
3. Select your domain from the dropdown list, then click Add Domain
Now you’ll be able to add mailboxes to all of your domains.