Running a low volume Exchange server on a dynamic IP account

I don't THINK that doing this would violate any terms of service (though the mods should correct me if I am wrong on that.)

The more pressing issue that you MAY run into, is the fact that other mail servers are sometimes (often?) configured not to send/receive mail to/from another mail server that is hosted on a dynamic IP address. Indeed, Comcast (and other ISPs) publish lists of their dynamic IP range blocks specifically for ths purpose. The reasoning behind this is to cut down on spam; many viruses will infect machines and set up rogue mail servers specifically to send out spam. Since the vast majority of "legitimate" mail servers are hosted on static IPs, the general assumption by some is that if you are hosting a mail server on a dynamic IP, you must be an infected bot SMTP server.

That said, there are some who report hosting mail servers on a dynamic address with no problems. As always, YMMV.

Additionally, I am not sure that Comcast will allow you to set PTR records for a dynamic address. This is another spam-avoiding procedure that many mail servers employ; if another mail server does not have a requisite PTR that points back to the domain name, mail to/from that server will be refused.

This is a big concern for me and I haven't recieved an answer from the dynamic DNS services that I've contacted about how/if they handle these issues. 

Just FYI, a dynamic DNS service will not be able to do anything about the PTR issue. Comccast controls that.