Hosting other mail domains

We already allow people to point their domain at us for web hosting, what about email hosting?

There are setups that can do virtual domains+users in such a way that users themselves could add their own aliases and manage their domains themselves. 100 different ways of setting up exim+dovecot+roundcube+whatever to allow this, so lets argue about how to later, and whether it's a good idea/viable now.

In terms of cons, I wouldn't want us to be the home of spam, in terms of sending, would we allow people to use silver to send email from their domain or just receive?

It's something worth thinking about