After making my server sign outgoing email I started to wonder what the benefits are.

This is the opposite of a previously asked question.

Naively I see two benefits:

1. We can throw away all emails which don’t carry a valid signature: Wrong! Mail forwarders (like Mailman) will produce emails from someone at «domain which signs» which are not signed correctly (in their forwarded shape).

2. We can skip spam checking on signed email: Wrong! A spammer can send a single email through e.g. gmail.com and then resend that email as-is (w/o changing headers) to a million people.

So what are the selling points of DKIM?