Custom Email Setup For Domains

Hi - I would like to know if there is a plugin or a way where I could setup email accounts for my users that join my multisite network? What I would like to do is allow them to create their own email addresses depending on their domain name that they have mapped with.

For an example:
user1.example.com = user1@mappeddomain.com
user2.example.com = user2@mappeddomain.com

How can this be done? Thanks!