BeatWare’s Mail-It utilizes your central People database as its address book, like a good BeOS citizen application should. In fact, the interface onto People files Mail-It provides is so good you may prefer it to the People app itself.
If you want to create a Person file from an incoming message, just single-click the sender’s e-mail address in the message header. A new contact form will appear, with the sender’s name and address already filled in. Add any additional details, click OK, and a new Person file will be written to ~/people.