Stating false accusations to a third party which defames your character is illegal. Though, you'd have to think about what lead him to this conclusion. Was it just something he completely made up, or was it based on something you posted which may have led him to believe this? If he was honestly under the impression from something you said that you had drug problems, it probably would be hard to prove that he did it knowing it was a lie and with the intent to defame you. You also have to look at how the email to your mom was written. Did he state you were using meth, or did he merely state a concern about it? The two are completely different. Either way, it's a pretty low thing to do, to say something like this to someone's mother. If you find out more about who he is, you could definitely file a small claims case, sue for the maximum allowed. But make sure you have copies of all the emails and any evidence proving that he sent them and any previous dealings you may have had with him. If you cant figure out more information about him, ask your local court, they may be able to tell you how to go about getting a subpoena for his ISP to release his information. I'm really not sure how that works, but maybe it's possible.
So your options are pretty much to pursue it or to just move on, but like I said, if you pursue it, you need to be able to prove that he knowingly lied and showed intent to defame your character. If you don't choose to take it to court, I doubt theres anything an ISP would do about it. They're not in the business of handling personal disputes...
