I'd like to host emails for domains only. What control panel would you guys recommend? Currently I'm using Cpanel.

I like Cpanel due to its design and capabilites but you could try the old fashioned way and use Windows Server 2003 Email features.

You could look into providing Hosted exchange if you really want to specialize with Email hosting.

In terms of control panel though, cPanel is a good solution. The only thing I would say is that if you are using it Purley for email then you could probably make more efficient use of your resources with a lighter control panel. DirectAdmin could be a possibility although I am not 100% sure whether it has email-only functionality.