It would be nice to have the ability to define an email domain in the media type its configuration that will be enforced to be used.
This would allow to restrict where emails might be send to by users.

Real life examples could be:
Media "Email" :company.tld
Media "SMS by Email" :email-to-sms-by-provider.tld

If the user defines one of these media types a possible behavior might be to truncate the address beginning from an eventually existing '@' and put the predefined domain at the end.