I use dynamic DNS also. I don't know if it will work for you. Dynamic DNS just keeps your DNS record updated when your IP address changes. It does this in one of at least two ways. If your router supports it (and many do), you can just set up your router to update your dyndns record when your IP address assignment changes. If not, you can download a small applet to run on your computer in the background that will do the same thing. (I happen to use ddclient on a linux box, but they also have windows clients.)

Dyndns allows me to make connections to my home computer no matter when the IP address is. Though you might be able to use your own dns hostname with this service, I use one provided by dyndns.org. (I pick the name, but it's their domain name, as in lihprem.dyndns.org)

For smtp, if your isp is doing authentication by numeric IP address, dyndns won't help. However, if he can do the authentication by hostname rather than by a fixed numeric IP address, dyndns might work for you.

-David