Hi Del,
Check that the External IP address of your server (find it in Server Manager -> Configuration -> Review Configuration) matches the dyndns IP address record. If you update the dyndns IP address from one of your network clients, it sometimes auto-detects the IP address of any proxy that your cable provider uses, not the IP address of your server.
And, as Ian said, check that your server's domain name is the same as the dyndns domain name you are redirecting to your SME box.
HTH
Andy