Prosites domain mapping and email

Hey, just a quick question.

I plan to limit domain mapping to only paying users with Pro Sites. My question is, how can users have there own email? Is this something they set up through there domain name provider?

Scenario, User signs up for my site, and buys a domain name through whatever hosting site, but they don't buy hosting as my site provides that. Is the user still able to set up a professional email for that domain name?

Ex. User's domain name:
User's professional email address:

Is this possible or do they have to have hosting to achieve this?