Membership and emails for non completed subscribers

Hi,
I am in the process of setting up membership and have a question.

I really like the fact that users submit login details before they pay - had a bunch of user problems with my old set up which sent out an email to create login after payment.

What I want to try to capture is the people who create the WP registration, but don't complete the subscription. So basically, if someone comes to the site, creates the username and password, but doesn't pay. I know that they won't have access to anything that is protected, but I would like to have a way to send out a targeted email to this specific user (not people who complete the subscription) in order to encourage them to complete the subscription process.

Any suggestions on a set up that would accomplish this?

Thanks,
Matt