I'm looking for free newsletter software for my website. Basically, I just want to put a form on my site that lets users enter their e-mail address and then send e-mails to these addresses from time to time. Is there any free solution for this that isn't too complicated, but still has all the important features like unsubscribing and so on? Handling bounces would be great too, but not necessary.

The programming language should preferably be PHP.

Thanks!