Question : New domain 550 5.7.1 Unable to relay

I have SMS (symantec mail security) for SMTP accepting mails and forwarding them to my Exchange server. Everything works fine but I now have to receive mails for another domain on that same Exchange server.
I have done the following:
1. Add required DNS entries such that mails with new domain are directed to my server
2. Add routing for new domain in SMS configuration
3. Add an additional smtp email address to my existing email account
But I cant receive emails sent to the new email address from the internet
On my SMS for SMTP I get the following errors
550 5.7.1 Unable to relay for xxx@xxxx
Mailbox unknow or not accepting mail
What have I missed.
Thx to advise
Brian.

Answer : New domain 550 5.7.1 Unable to relay

Within Exchange System Manager, open Recipients open Recipient Policies, right-click on your Default Recipient Policy and choose Properties.  Click the Email Address Policy tab and click "New".  Choose "SMTP Address" and type the name of the domain in the format @domain.com.  Click the checkbox for "This Exchange Org is responsible..." and click OK and OK.  If it asks you to apply the policy to all addresses affected, so NO.