If you are not 100% sure that the email makes you certain that he is using/buying drugs then make sure first that he is engaged in these behaviors. If the email is certain then approach him about this. There is no way to do this so that he is not challenged. He is going to be upset. Dont' get caught up in whether or not you should have read the email. He is probably going to bring that up first as a defense. After you have stood your ground then ask him everything you need to ask in a calm and nonconfrontational way. Tell him that you read the email and very concerned that he is buying drugs. You are concerned for how this will effect him and his health. You are concerned for how this will effect your relationship. Present those issues in that order. If you present that you are concerned about him (and your relationship) you shouldn't get the reaction you are afraid of but even then you run the risk that he is going to be defensive. When someone is engaged in criminal activity you always run that risk. But you can't ignore it and hope it goes away. Hopefully these tips will give you a chance to discuss this. If he blows up give him a chance to cool down and return to these suggestions. Don't engage in defensive or irate behavior.
Hopefully he will be open to seeking help for whatever behavior he ends up disclosing. He knows that this will effect the relationship so just be calm and consistent.
If this has been helpful press accept