I used to have a lot of problems with email, but finally got it right thanks to one of the GTD Connect webinars, I think it was called precisely 'Dealing with email' or something like that. I don't know if the offer is still available, but they used to offer a 2-weeks free trial... if it still is, give yourself a treat and check those webinars out; they really take your GTD practice to a new level.
My email load isn't even close to yours, but here's how I finally got it nailed, fwiw:
I have created two folders, @project material and @waiting for. When I process my inbox, for each mail, I decide if it requires an action from me, and if it does, I determine which action is, move the mail to @project material so I can refer to it later, AND write the action in a different list, in my case @computer (this was my problem for a long time: I used the email itself as the reminder of my next action, but the next action is not self-evident by just reading the email, so next time I saw the emails I had forgotten what I had to do).
In your case, having so much mail, maybe it would be useful for you to make subfolders for projects within @project material. I have done it sometimes, when things become complicated, but I usually don't need to.
Later on, when I'm ready to do the action, I open @project material to get the mail I need, and once I'm done I move it to another folder called Archive. If I have to recover something later, text search works like a charm in my case.
@Waiting for is for mails from which I expect a response, and also, for example, when I purchase something online and I'm waiting for it to arrive (to refer to the mail in case there are problems with the deliver, etc). Again, I write down the due dates in a different location, my 'waiting for' paper list, and it gets reviewed regularly with all the others. These two folders also get checked and updated during the weekly review, so they don't become a 'black hole'.
Hope you get something useful out of this. It seems simple once in place, but email was one of the things in my GTD setup that took me most time to take to autopilot.