How to send activation emails to deactivated users?

Hi,

I want to implement a feature which enables the user to enter email address in a form, and the form will send an account activation email to the submitted email address, if the email address belongs to a de-activated user.

Is there an app which can achieve this? Or do I have to build a custom app? If so, how do I go about implementing it?

Would greatly appreciate any guidance on this.

Thanks