Sure they can. For sending e-mail, all you need is an IP address. It would help if the reverse DNS is set up correctly, and that you claim this same name in the SMTP dialog, but this isn't required.
Yes, I know they can today. My point is that with a registrar based system, they cannot, because they cannot be registered as valid mail servers.
For receiving mail, all you need is a domain name, which has a set of advertised MXes. Those MXes could point to mail servers operated by friends of yours who might use UUCP, or some private routing method to send your mail to whatever your current IP address is. Those MXes could even point to your own host/domain names, and the mail would be deferred until such time as you re-connect with your dynamic DNS provider and update the IP addresses for these names.
Correct, but MX's (mail servers) have static assignments, unless you change DNS every time. Running MX's on dynamic IP's to receive mail would be quite silly. -- Robert Blayzor, BOFH INOC, LLC rblayzor@inoc.net Exclusive: We're the only ones who have the documentation.