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You should not use NAT in ZS. If your clients in the internal network use x.y.239.121 as the default gateway there is nothing else to do. Most like the problem is that the ISP is not advertising the /29 subnet that he has allocated you, or is expecting from you to advertise it via RIP or OSPF.
A quick way to check is to try and traceroute from the ZS a public server such as http://www.yahoo.com and then try the same from a internal PC. If the traceroute stops at the ZS then most likely it is a problem of your ISP not returing the traffic back to you.