Ask HN: Any free e-mail clients with scheduling and tracking?

I once tried Polymail when it was early in their development, and I thought the UI was pleasant and the features were great. But at that time, it was still buggy and I had a hard time integrating my work and personal e-mail accounts smoothly, so I gave up.

But now I have use for such features but can't find anything convenient/free that does this.

Do any of you use a client or tools that do what I'm looking for?