Ask YC: Email marketing tool

I am looking for a low-cost email marketing tool that I can use to start sending newsletters and messages to potential customers. I want to find a reliable provider that has high deliverability, but that isn't too pricey. I'm a start-up, after all.

I considered building this myself, but I'm guessing that there are lots of good products already on the market. Can anyone suggest something that they've used?