1. I have one inbox. Everything goes to the same place (accounts either forward to Gmail or I've actually set Gmail up to reply from those accounts).
2. I only check email when I can reply to it easily. Unless I'm waiting for something specific, I try not to check email from my phone, because it's a recipe for getting an email that requires a length reply that I don't have the time to give on my phone. And that just stresses me out until I can reply appropriately.
3. I don't use preview windows. It's too easy to only get half the information and miss important stuff. When I used Outlook for work, this happened way more often that I would have liked. I thought it was a feature, but it turns out it wasn't helpful at all. It made me browse email more than read email.
4. When I do check email, I start at the bottom and move my way up, going through email one at a time in the order sent. It's the only way to know I haven't missed anything.
5. I know my hot keys. By not using a mouse, I fly through emails much faster.
6. If I can reply quickly, I'll do it right then. If I need to think on it, or know it will be a long reply with lots of edits, I'll star it (a Gmail feature), so I don't lose track of it. Alternatively, if you have folders, just move it to an Action folder.
7. Two or three times a year I'll unsubscribe to newsletters I'm not using anymore. It's amazing how easy it is to get onto newsletters without really trying.

Having email sorting/filtering would be anti-GTD if you use it to avoid decision-making, but not if it's just for evaluating what kind of attention to put on something. Using colors for certain people's emails in  Lotus Notes (as I do) would also be "anti-GTD" if you never dealt with the non-colored ones. We're not officially endorsing or recommending this.  Just saying it's something that you can make work.  - David Allen