We have seen several adresses posted around the net but we have no idea if any of them work. To our knowledge doesn’t Eva have a fan mail adress.
But if you want to make it a try use any of these adresses:

Eva Mendes
Management 360
9111 Wilshire Blvd.
Beverly Hills, CA 90210
USA

When writing, please include a properly stamped and self addressed envelope (US) or several International Reply Coupons (international) if you are hoping for a reply. But please keep in mind that Ms. Mendes is a busy lady – don’t get your hopes up on hearing back, atleast anytime soon. And if you have any luck and gets an answer from Eva, let us know!