Ask HN: How to handle emails from customers?

I'm an independent software developer. Most of the emails I get are feature requests. When I implement a new feature, I will search my email archive and contact everybody who asked for it (sometimes months after they contacted me).

Right now I use OmniOutliner to keep a list of requests, and manually add names of people who requested it.

Can you think of a more streamlined solution?

I've looked at issue trackers, but most of them are horrible to use and often require people to sign up on a website rather than just sending an email.

I've looked at CRMs, but they seem to be focused only on sales.

Is there a tool that will help me stay in contact with customers even when I'm not trying to sell them something?