I have 6 pieces of email in my inbox right now, down from somewhere slightly south of a thousand a few days ago.
Much of the mail that was cluttering my inbox was simply waiting to be filed or deleted. I delete very little email, only stuff that's really junk. The rest... goes off into a vault where I may delete it after a few years if for some reason it's bothering me that it's there.
I have an aggressive set of mail filters which pre-sort many things for me, and I have a few smart mailboxes which help me find all the mail I've received in the last day, or mail that's older that I haven't looked at, regardless of which mailbox it landed in.
But still with all that, it's been easy for me to let email get out of control. I'm traveling, I'm busy, I'm sick or I'm just not feeling like dealing with it, and very quickly it gets to the point where it will never be dealt with.
And what I've just realized is that a big part of the problem is that I treat my Inbox as a Todo list. When a piece of email needs me to do more than just file it, if I don't deal with it immediately it often ends up just sitting there. Waiting to be dealt with. Someday - likely never. However, its presence nags me and I continue to feel bad about never having dealt with it.
My Inbox is a very poor Todo list. It doesn't really give me any tools for managing the stuff I'm trying to do. Like due dates, or contexts in which I can do things, or projects that the items are related to. No, I use a Todo list manager - Omnifocus - which excels at that kind of organization. Omnifocus even gives me a solution to this problem: enter an item that I need to do and simply attach the related email to the item. When I want to refer back to the email, I can click on the the attachment and the message will open (as long as I haven't deleted it).
Omnifocus has a couple of other tools to help quickly get email actions into its own inbox, but I haven't started using them yet. And honestly, I use Omnifocus in fits and starts. It's a great tool which I unfortunately haven't built up the habit to use reliably yet, partially because I don't need a todo list manager every day.
I have 6 messages in my email Inbox right now. These all require some level of me doing something - writing a thoughtful reply or possibly a bit more. I've written several replies to messages a year or more old, basically apologies for letting them go, realizing that it was no longer relevant that I hadn't replied but that I felt that I was rude and needed to say I was sorry.
These last 6 are the toughest. It's hard to make the decision that I should just make a note in Omnifocus saying that I should reply to a message, especially one that for whatever reason I've been having a hard time writing a reply to.
Philosophically, one of the most common causes of suffering (mild or intense) that I see is when people confuse one thing for another. It may be food for solace, friends for family, their idea of themselves for the reality of themselves, or many, many other things. Even an Inbox for a Todo list. If I can remember to use email for email and my todo list for reminders, I may be able to avoid getting one thousand messages behind again.
[As a side note, my Inbox is also not a repository for all the little things I should remember and might forget. Evernote is much better at that and a more appropriate solution.]
✓ Use Omnifocus for my todo list and Mail for email