How are you getting your mail to these domains currently (if at all)? The usual solution is a mailserver somewhere that will accept mail to these domains and you tell it to forward anything it gets. You then advertise your mailserver with MX records at your DNS server.
You can run your own mailserver on your Ubuntu machine or your ISP may provide one (if you're setting up domains, I presume you have a static IP account which usually comes with a mailserver) or you can just buy the service at any hosting company of your choice. If you use your ISP's or somebody else's commercial mailserver, they'll typically provide a web interface to help you set everything up.
But Googling just now, I notice another new alternative is to use Gmail to host your email domain directly. I haven't tried it but they apparently will do it for free.