Email Integration with wordpres mu

Hi

I just thinking to create an email integration with my wpmu. If somebody registered such user.example.com so it also create an email user@example.com and if they register a multi domain such example1.com, they will be an administrator of that domain, so they can add a new user for domain example1.com.

Is there any plugin or other way to create this ? And which webbase email that I can use ?

Any possible scenario ?

Thank you