There is one way to get rid of your 3rd party app and work with a DHCP Address.
There is a website out there called ENOM.com that allows you to create a DCHP MX record. You load a DYN client on to your server which reports your address back to ENOM then they update your MX record just like A Records.
I have my server (Exchange 2003) working in this fashion.
Hope that helps,
Zeroinfin