If you think that you might need to take some action in the future, put it to Someday. If its purely informational, then its part of the project reference material.
I'm involved in many projects on which I'm cc'd on every email. Makes for a pretty busy in-box. I've been handling this by creating a Reference folder. Under Reference, I have Clients. Then 1 folder for each Client. If I get a project email that's really an FYI for me, I file it in the Client folder, and assign it to the appropriate Project. In the Client folder, I've created a View so I can see everything in there by Project. (Let me know if you need instructions on how to do this.) Because I need to keep everything pretty much indefinitely per our company standards, I put all closed tasks in this file, too. So, only things I really need to take action on are in my Tasks, and all of the project history ends up in the Client Reference folder. If I need to find one of those FYI emails, I always know where to look. Eventually, when all of the tasks are complete, the entire project history will be filed neatly in the Client Reference folder. Seems to be working for me.