This really opens up a nasty can of worms. On the one hand you want the provider to deny spam mail to your account. On the other, you want privacy. Who can say that a piece a mail from .ru is actually spam? Maybe you requested some literature from a company there. Is the provider now to act as your Mommy and Daddy by reading through all your mail before you get to?
What about those companies that hang directly off the internet -- their provider is just supplying them bandwidth and nothing more. The company has their own mail and web servers. Is the provider in this case authorized to inspect company confidential e-mail when they're not storing it on their servers?
I didn't notice this reply in my comments, sorry for the delay. My idea is to stop spam at its origin, not the recipient. Spammers are quite easy to identify: thousands of messages with forged headers are sent from a single host. Even if the headers are legitimate, numerous abuse reports should give the spammer's provider reason to investigate. If an ISP doesn't take action, their provider should stop their ability to send mail.
Is the provider in this case authorized to inspect company confidential e-mail when they're not storing it on their servers?
Inspecting the contents of the e-mail wouldn't be necessary, all that is needed is the header information, which isn't confidential.