To use leadingedgesystems.net for your dialup e-smith machine will be a multi-step process, but it's quite do-able. You've already registered with dyndns.org, so presumably you have a domain with them something like yourdomain.dyndns.org (or .mine.nu, or whatever). You configure your e-smith server to use this domain, and use dyndns.org to provide dynamic DNS service.
Once the e-smith box is configured this way, you'll probably want to test it, preferably from someplace other than home. When you're sure it's working, go to zoneedit and set up an alias for leadingedgesystems.net and
www.leadingedgesystems.net to point to yourdomain.dyndns.org (or whatever). That's all you need to do.
And yes, you can do mail and web serving over a dialup connection with a dynamic IP on e-smith; I've been doing it for over a year (until a couple of weeks ago, when I finally got DSL). It's slow, and the occasional disconnects are frustrating, but it works reasonably well.