How does detecting the emails were fake address the DMCA claim on his YouTube channel? It’s my understanding that YouTube won’t intervene in DMCA claims even if the claims were false.
the most important thing here seems to be that the fraudulent claims were retracted by the person who sent them, and the statement from nintendo that the claims were not legitimate.
but it's not true that youtube won't do anything about false claims. both the DMCA and youtube's pseudo-dmca takedown process care that about the identity of the person making the claim. if you claim "i am an authorized agent for Nintendo of America" and that's not true, then they'll throw out your claim. as long as you are who you say you are and are an authorized representative of the company you claim to represent, then google tends to trust you even if you're claiming ownership of content you don't actually own.
