gbentley:
How are you updating your Dyndns.org - or what client are you using to update it? Since it is not the SME box, is it your Router or Firewall? Many newer routers come configured with a dynamic DNS client. Or you can just use a firewall like smoothwall or IPCop. I forward several domains through multiple smoothwall boxes (mostly with no-ip.com). But I use real domain names with no-ip.com as the nameservers (buying their service).
Setup a firewall or router with a dynamic dns service, then forward smtp, and https through to your SME box. Test away, and let everyone know your results.