Unique mail server and separete mail quotes

Hello,

I see a lot of hosting companies with cpanel offering plans with like X mb for site+dbs and Ygb for emails. For example, 500mb for site+dbs and 10 email accounts with 1gb each. I wanna offer something like that, but i think cpanel doesnt allow it. If so, how do they do that?

What if i configure a server and centralize all emails from all my servers in it. I mean, i create a regular account in a server (with 500mb) and creates it again in another server (and sets it to have 10gb and allow 10 email accounts). So i set only the mails to use that "mail only" server (that will have cpanel). Is it possible?

And can i use that server for centralize all email accounts from a lot of servers, cant I?

Thanks,

Geraldo