Originally Posted by ajnz

Dynamic DNS will not work in this case - most mail daemons use IP addresses, not host names, for their access control. In the unusual event one does use host names, it'd also be cached, so if your IP address ever changes rapidly, the mail server may take time to catch up.

I'd second ScottC's suggestion to use Google for domains. The other thing would be to ask your real ISP to implement SMTP authentication (with TLS, of course), which bypasses the need for a static IP address and allows you to authenticate to their SMTP mailserver using your username and password.