Please stop replying to his emails.

Why would you even do that in the first place, knowing how unstable your ex is!?

No point in blocking his texts and calls if you're still going to email him, you sent him mixed signals, no wonder he flipped out about new guy.

Make sure the police see copies of the email threats, that way they know exactly what they're dealing with, and for Christ sake, stop stoking the fire by returning his emails!