Whoosh is an OS X app for sending out emails via Amazon SES.

If you're not familiar with SES, it's a simple service and API from Amazon that makes sending out mass emails (like newsletters) to thousands of folks super easy. I've been using it for a couple of years from a custom Python script I wrote, but it looks like Woosh would have been a better solution.

Why use SES over anything else? It's amazingly cheap, and it was well worth my time to switch from Campaign Monitor to it.