[jeffbee]

You are inventing imaginary requirements for email that do not exist. The "From:" header is not part of the envelope. The "From:" header is protected by the DKIM signature. This message is from youtube.com. You received it via some other relay. Nothing about that is weird. People love relaying email all over the place.

[kazinator]

Do you have a hypothesis about why someone would be re-sending these YouTube messages? One possibility is that the original To: recipient of this message is an bad actor's address. Maybe some small percentage of users will contact that address, and thereby get harvested.

[jeffbee]

I think they're trying to goose their sender reputation, but I don't know if that is going to work.

[kazinator]

Interesting; I actually thought about that too! But there are ways it could backfire, like people reporting this junk to blacklist sites. Spamcop doesn't care that what you sent is a copy of a valid e-mail from YouTube with good DKIM.