Associate an authenticated domain with a given user, for up to five domains.

**This endpoint allows you to associate a specific authenticated domain with a subuser. It can be used to associate up to five authenticated domains.** This functionality allows subusers to send mail using their parent's domain. Authenticated domains can be associated with (i.e. assigned to) subusers from a parent account. 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 d...