auto email and e-newsletter plugins

We are trying to setup a network of websites and allow each of these sites to sign up customers .. and consequently deliver follow up emails and e-newsletters to customers that register on their site .

We have discovered that a user can only register with one site within a network, if they attempt to register with more than one site .. we get an error regarding "that email is already in use"

We want a user to be able to register with multiple sites across the wp network .. as if each of the sites within the network was totally unrelated ...

each site will be owned and operated by a different company .. similar to .. where each site is considered as a standalone entity .. is this possible ?