"Tu email me hace feliz"... if you are talking about one mail... if it's a general feeling it'd better say "Tus emails me hacen feliz" or "me hacen sentir bien"
Your second phrase could be translate literally as "Cuando veo tus emails, me siento feliz todo el tiempo" but I'd say something similar as "Siempre que veo tus correos, (or emails) me siento mejor"