Most people don't really understand the CAN-SPAM Act or spam laws. As long as the header and other information is not misleading in the email, and you provide for an opt-out, you are likely not in violation of most spam laws. That's how we all get our emails added to newsletter lists, even though we didn't "opt-in."
I don't know where all fo the newsletters come from. But today, I searched through my inbox and unsubscrived to every newsletter that I thought was legitimate, although unwanted.
Another change we made is to stop copying so many internal attorneys in on the email that goes around for clients. Now, we only copy the attorneys who are involved in a particular project on particular emails. This has reduced the amount of total emails to the inbox by approximately 85% in our firm. While it's great to know everything that's going on in all cases, we easily can access the case management system in order to research a particular file.