At a certain point, you will clean up your email. For some people twenty is too many. And for some, it’s five thousand. Different standards for "stuff."
These standards are very powerful unconscious drivers of your behavior and permitted experience. You may consciously think you’d like to keep a neater house, or process your email more regularly, but if you don’t change the set point of the real standards you have about the amount of out-of-control-ness you actually will tolerate, they will slide back in spite of your best intentions. Pit your willpower against your unconscious cruise controls, and guess where I’ll place my bets.
If the good fairy visited everyone you know and work with right now and magically dissolved every email sitting in IN, within days the number would be back up to the comfort zone of the individual. Some people would have twenty, some three hundred, and some two thousand. Even people doing the same jobs, at the same level, with the same amount of input.
For email, it’s actually less effort to maintain it at zero than to maintain it at three hundred. The decision about the next action is still unmade for much of what lies in IN (hence it is still "stuff," i.e. something in your world for which the action is still unclear). Every time you even slightly notice that email again and do not dispatch it, it wastes energy. As soon as you allow indecision on the front end with any of your input, you have broken the code and it will mount up all around you.