Default Email domain for Exchange 2010

I have a scenario similar to another question but it doesn't help me. I configured my new server 2008 domain to be corp.acme.com - so now email addresses want to default to user@corp.acme.com
I have tried to create a new email address policy so the email addresses can be user@acme.com, but I get an error. The error reads "The SMTP template 'smtp:@acme.com' is invalid because it references a domain that isn't configured as an accepted domain for your organization."

Thanks