How to offer email accounts to MU clients?

We are working on offering WP as a service with this plugin, but we are wondering how to offer these clients email accounts as well, but in an automated way.

We need to park the domain of the client on top of the main MU domain, so we won't be able to create a cPanel account for that domain. And Google Apps is not offering free accounts anymore.

I've considered Rackspace Email since they could be automated by this plugin, but that will involve charging $2 / month per email account.

Any ideas or feedback is appreciated!