Don't tell him who did it, just that it has happened. Or forward them to his own e-mail from now on.

He is a cheater, and they'll do ANYTHING to keep from getting caught in thier lies. My husbands' ex is a liar so I know how much they'll squirm to avoid getting caught... bending the meaning of words, the truth, misdirecting you to try to get you off the topic, and plain old making stuff up...

The first time around, he probably knew full well that his girlfriend was doing it but blamed it on your daughter and then ran to the girlfriend & demanded that she grow up and start acting right. The girlfriend probably said it wasn't her & she had no clue who could have done it (the way liars do), and he, so anxious to believe her, maybe thought it was some friend or maybe it WAS your daughter, in his mind.

But this time... he'll know. You don't need to tell him who is sending the e-mails, just that you're receiving them and that whoever it is, there is OBVIOUSLY someone who is following his movements and sending anonymous e=mails to you, and not that it matters to you, but he might want to watch his back since he's got people who are pretending to be his friends, who KNOW what he does in his private life, who are reporting this stuff to you in this weird, anonymous way.

You don't have to say a word about who you think it is and if he starts accusing you of falsely accusing the girlfriend, just say you don't knwo who it is, and at this point you don't really care any more, but at the risk of burning your source of private information about him, you just thought he'd ought to know that someone is acting NOT in his best interests and you don't much care how he handles it.

And offer to forward the e-mails along to him so he can trace the IP address or whatever, if he needs to do that to catch the culprit.

He might take you up on the offer and will give all pretense of following up, but when he thinks he's got you thinking it's some buddy who is playing a practical joke or something... he'll run back to his girl and REALLY give her hell for it.

Either way, you'll be very likely to not have to deal with the stupid e-mails again.

On the other hand, if you want to, you could get some administrator to trace the IP address from the e-mail, claiming some version of fraud or harassment (because it's surely one or the other), and the server responsible for allowing it to happen might prosecute her for it.