If you’re using Mozilla Thunderbird to read your email, then have a look at a new extension: EagleEye

It allows you to re-discover old friends (= people with a private email address to whom you haven’t written for several months) or keep track of your TODOs (= people with a professional email address where the last email was sent by the other person and not by you, so that you should probably reply).  And MUCH more.