SME probably tries to advise dyndns.org, but they aren't accepting it. To verify this, go to the log file viewer and look for dyndns.org in /var/log/messages.
Probably what you need to do is edit /sbin/e-smith/dynamic-dns/dyndns.org and change this line:
system=dyndns\&hostname="$DOMAIN"\&myip="$IPADDR"\&wildcard=ON
to this:
system=custom\&hostname="$DOMAIN"\&myip="$IPADDR"\&wildcard=ON