This is a somewhat involved configuration, you may want to think twice about doing it yourself if you have no experience with this kind of thing. You have the potential to break your email connectivity. That said...
If I understand correctly, your new Barracuda is going to be a host (in your DMZ I assume?), and you wish to open the firewall to allow these ports into the barracuda's NAT address which you will also have to create?
I will also assume you have existing internal email server(s) that handle your in and outbound mail now?
As part of the process, you will also have to repoint your DNS MX records in public DNS to the Barracuda NAT, and set up your email server to send/recieve mail through your Barracuda.