Can you draw on the outside of an envelope and mail it?

Yes! You can draw on the outside of an envelope, you can write on the outside of an envelope, and you can even glue, paint, or decorate the outside of an envelope AS LONG AS the address is CLEAR and the stamp is CLEAR. Don't put anything bulky on the outside of the envelope (ex. buttons, loose strings or ribbons) otherwise the postal service won't send it. But otherwise, you can do whatever you want to an envelope!