There are two types of email:
1. Email that you need to attend to because it directly affects you. Eg. Email from your accountant telling you the ATO is about to sue your butt for tax evasion.
2. Email that someone else needs you to attend to because it directly affects them.
Now, type 2 has two sub-types:
2a. Stuff that is critical. Eg. Email from your boss asking for your billing sheets. Sure, somewhere down the line, it's going to affect you directly in the form of a lost commission or something, but more importantly, it's going to affect someone directly enough that they are going to follow up with enough regularity (including cc'ing the world) as to make you look like a complete slacker.
2b. Stuff that is non-critical. Eg. Email from you boss asking you to put "together" a quick "report" about how many customers you think use random product x. Now, this type of email is a classic Inbox stuffer. It may not come across in this exact form, but more times than not, it's the type of email that someone doesn't really care too much about in terms of response/action, but more along the lines of "well, I've dealt with it", also known as "passing it on".
Now for most folks, and me too for the first year of my job at Microsoft, most of the emails that were in my Inbox, under the category of last week or before, were type 2b emails. I spent most of my productive time attending to these, only to find that most people didn't really care two hoots about them.
Then, one day, I started just deleting them. And you know what happened? Nothing. No one ever pinged me back, unlike if I had just deleted a type 1 or 2a email. So then, as a matter of course, I would go through at the end of the day, and rank my emails. Anything 2b, just got deleted, everything else got actioned.
Or, you delete all type 2's, and wait for the 2a's to show themselves (trust me, you'll know which ones they are ;)).
To this day, my strategy has paid off. I'm yet to have a deleted 2b email come back to bite me, so for those out there who are struggling under the weight of a bloated email, delete your 2b's, you'll feel better for it!