Best wordpress integrated emailing software?

I’m using feedburner for my emailing, but it’s buggy (sometimes sends old posts) and doesn’t seem to have the option to change the sending intervals (such as a once per week newsletter).

What is the best wordpress integrated emailing software? I don’t care if it’s free or paid, I just want something powerful that I can tell it how often I want to send, and what I want it to send.

Any suggestions?