Associate an authenticated domain with a given user.

**This endpoint allows you to associate a specific authenticated domain with a subuser.** Authenticated domains can be associated with (i.e. assigned to) subusers from a parent account. This functionality allows subusers to send mail using their parent's domain. To associate an authenticated domain with a subuser, the parent account must first authenticate and validate the domain. The parent may then associate the authenticated domain via the subuser management tools. [You can associate mor...