Returns a paginated list of the user's whitelabel/sender domains.

Sending domains are shared by marketing campaigns and transactional email. Every Domains endpoint accepts a paid-plan API key or a transactional key that carries the transactional:domains scope. A transactional key without that scope gets a 403 in the transactional error shape (insufficient_scope), and one on an account without transactional access gets feature_not_available.