I've started going through my year-old emails again, and I find it to be a tremendously rewarding experience. One year ago today, I got a tip from a friend about a recruiter who eventually ended up landing me the job I have today. I think I'll write her a note and say thanks again. I also got an email informing me that a good friend of mine was expecting his first child. Looking back through your old email gives you a chance to reflect on what was really important and what was just noise. It's like keeping a diary without doing any extra work.
Of course, this year-old email review requires that you actually keep all your email, and that's not something everybody does. I think it's a great idea, but then I have somewhat unconventional ideas about email anyway.