eli5 How can an email be traced?

Someone keeps sending death threats to my sister's store through a mail. ru email account. I tried going through the email header and the IP address is in moscоw. I tried sending myself and email through mail. ru and it has the same moscоw IP address so its obviously the server IP. There's no way a Russiаn mail provider is going to cooperate with a subponae from the US so is there anyway to trace the sender? yes law enforcement …

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