I think I have this right, but am looking for a yes or no answer to be

I have a Barracuda Spam appliance on it's own external IP address.
@mydomain.com points to it.
I am also getting spam mail coming into my mailservers own external IP
address, bypassing the Barracuda completely.

If I set my GWIA Class of service to Prevent incoming messages, and
then add the IP address of the Barracuda as an allow exception.

Will that 'solve' my non filtered spam problem, and still allow all of
the legitimate email that Barracuda lets through?