How to offer e-mail addresses on demand?

I am using wordpress multi-site with pro-sites, marketpress and multi-domains...offering shop and blog sites. I would like to offer e-mail addresses (on-demand) to go along with the sites, just as wordpress and edublogs do. Is there a plugin available for this that I am not finding, or is it a custom program? Any assistance you can provide on what avenue to pursue to be able to offer this service would be greatly appreciated.