I'm looking for information regarding any email service provider backends that exist. Open source, paid, plugins, etc.
What I'm specifically looking for her is a back-end that could be built on to create a MULTI-USER service such as aweber, getresponse, constactcontact, etc.
If you know of such a white-label or open source backend please provide details.
Preferable something that interfaces well with Ruby on Rails apps, but not required.