I used to keep them all as my memory was not so good. I discovered I only have been going back to certain specific ones. So now I either delete after I respond to them or once a week the ones that I have responded too; also even more so I will delete the messages that I know I have sent back and gotten a response from. It helps keep the box at a minimum level. Even ones that are particular messages that I still want to keep, the box still stays way down below the limit doing this.

Good luck sorting through your box because not knowing what happens here when the box is full, in my other email accounts, I hate to miss messages because the box is full and they get rejected for the sender. Best wishes.