I installed this plugin, the author has put a lot of work into it. It looks great and is nicely laid out.

The only issue with the plugin – and it’s not really an issue rather a missing feature – the subscribe option. It’s not ideal to use emails sent to you without a confirmation, due to spam, robots etc.

If this plugin integrated with other mailing systems it would be a fantastic plugin, and probably saleable. The best approach would be to let users input the form code themselves, that way it should integrate with all mailing systems.