Craig Newmark's modest anti-spam proposal

Craigslist's Craig Newmark weighs in on the recent anti-spam/phishing debate. The Goodmail/AOL proposal, which Esther Dyson has written about, is built around a pay-to-send email certification scheme. The DearAOL Coalition rejects it as an "email tax." Here's what Craig proposes on his blog:

Here's what seems to be a great alternative to the Goodmail/AOL approach, though it needs to be fleshed out.

The intent of all this is to fight spam and identity phishing. Key is to provide acountability of email sources, by providing a means to digitally sign emails as from a trusted source.

I'd propose that any emails be passed through spam filters as suggested by Goodmail, if such an email:

is digitally signed, and

the digital certificate is from a recognized digital certificate certifying authority, like a state DMV, the USPS, or others to be named later

Such signed/trusted mail should be given credit in spam filter formulas AND be marked as signed/trusted by AOL et al.

That way, we get the benefits of spam fighting, and we get competition with Goodmail and others. I believe the software to do all of this already exists, and this can be done cheaply.

Well, what am I missing?