CiviCRM has a provision for creating a form for collecting emails from your web site and recording them directly into your PowerBase.
It involves creating a profile in your PowerBase, then copying the "html snippet" (a version of the profile written as html) and putting the html in your web site.
Here's the video explaining it. Also, pay close attention to the attached document that gives further details on how the profile should be set up.
File attachments:
Category:
Collecting and Managing Contact Information