Configuring Mail Domains
It's required to register a mail domain before creating an Approved Sender. Mail domains are managed in OTM: Business Process Automation > Power Data > Mail Management > Mail Domains.
See the Mail Domains help topic for details on how to register a mail domain.
Note:
Mail domains are accessible across all environments (Production, Test, Development). It's only necessary to register them from one of these environments and then they can be used in all other environments.