[ietf-dkim] BCC Recipients

[Hallam-Baker, Phillip]

> This doesn't help for BCC recipients at the same domain.

The only way to sign BCC in my view is to provide a per user signature
constructed by means of an HMAC.

For example message is "Hello World", Sending it to dot at dotat.at

So I construct a BCC identifier HMAC ("dot at dotat.at", SHA1("Hello
World"))

Or something of that nature. That means that the BCC recipient can
verify it was sent to them while preventing any To: or CC: recipient
knowing anything more than that there is a BCC.