I found it! Comcast has a spam e-mail filter option.
1) Do you want us to filter spam?
2) Do you want it put into a folder or deleted?
If you answer yes- deleted, any message that fails a reverse DNS lookup will never arrive.
About 45-50% of domains would fail this lookup. Anything hosted on a virtual server.
Best way around this- put a message on the registration that “confirmation e-mails may be filtered out by certain spam filters (such as Comcast or SBC). Please check your spam folder as this could be the case if your confirmation does not arrive.”