This answer points at RFC containing recommendations for auto-reply e-mails, and specifies a header to use.
Any new proposal will fall victim to the same opt-in problem to an even greater degree; if developers of auto-responders can't be bothered to follow this RFC, they will also ignore even newer recommendations.
But if I'm on vacation, I don't want to read your email right now. I'm just letting you know that your email might not get a reply, especially not any time soon.
Yeah, it seems like it's providing meta-information about the status of the email address owner rather than actually conveying information from the owner of the email address. Perhaps there should be a separate channel for such meta information.
