Just before the start of my faculty job I implemented inbox 0. Yep. Archived everything older than *three weeks* and divided the remainder into five categories. These are called “today”, “this week”, “this month”, “someday”, and “waiting”.
The idea is to sort emails in these categories if they take less than 2 minutes to deal with. “Waiting” means I have done my part and I am waiting for a response. Go through this list occasionnally to remind someone. See the potjies project entry last week... The labels are to be taken with a healthy helping of salt. And it is critical to remember that there are two ways for emails to retire out of their categories: they are done and dealt with OR they age out. Comments on a paper from 6 months ago? Too late. Better luck next time. Deadline passed? Ok fine. That is not great and feels worse but it an integral part of it.
So deal immediately, sort, deal with in order of urgency and then -critically- let it go. Sing the song if that makes it easier.