On the Legality of Spam Filtering

But my email fully complies with CAN-SPAM, the argument goes. How can ISPs be allowed to block my mail? I comply!

Here's how. Read section 8C of the CAN-SPAM law. It says:

NO EFFECT ON POLICIES OF PROVIDERS OF INTERNET ACCESS SERVICE-- Nothing in this Act shall be construed to have any effect on the lawfulness or unlawfulness, under any other provision of law, of the adoption, implementation, or enforcement by a provider of Internet access service of a policy of declining to transmit, route, relay, handle, or store certain types of electronic mail messages. 

Seems pretty straightforward, no?