Spam often appears to be coming from someones official website.
A huge number of domain owners don't know their site is being used for email spam until they get a returned email notice. Email headers need to be secured before you can blast the company the emails appear to be coming from. If google starts handing out the ban stick because of emails I suspect a large number of sites will get attacked by competitors trying to knock them out of the serps.
Hopefully they address this. Good move for google but now postini will also become the focus for many spammers, this won't stop them. Dozens of talented coders are likely already working on new ways to spam. Until more are caught and punished, spam is here to stay.
Maybe its time to offer an option to split email from domain names completely. A website without an email address or email server is the only surefire way to stop spam from originating directly from that site.
If every webmaster signs up for a gmail account, registering their business to that account, and use that for their business needs and then completely dismantle your website email system at the host level... spam (possibly) becomes googles problem.
A legitimate email campaign could be registered in advance to avoid it being confused as spam much like adwords screens ads.
I'm sure this has all been discussed before.