Getting a new customer can come at a high cost. Making a customer active, loyal, a brand ambassador,… is what really counts. This can be so simple, but so many companies do not even try to communicate with their new customers, let alone start a dialog with them.
Off course, you should target your new customers when it is relevant for them (See more on event driven marketing), but dropbox shows how simple this can be (no rocket science here): When you register for a new dropbox account and you do not use it for some weeks, you get a simple mail reminding you of the use for this simple but great piece of cloud software.
You probably cannot make it more simple, but it works. Great!
Ask yourself this question: what do you want to tell to your inactive customers, to your clients that have an issue, that are moving away from your service?