Hi Diederik,
Yup! We offer API connectivity exactly as you describe it. Here's an overview of our API:
https://api.rainkingonline.com/
You can use a Marketo webhook to call our API and send the email address when a new Lead is created. We can return all available fields in either XML or JSON format. We can also integrate our API with your web forms, so that when a Lead fills out your form you get all info on that Lead - delivered directly to your Marketo database or a dossier via email:
http://www.slideshare.net/ScottFerrebee/rainking-form
Here's a quick YouTube video I put together on this:
https://www.youtube.com/watch?v=XgT1fI42M0E
Happy to discuss any of this further - these are some cool use cases.