Comcast refusing mail

I saw a thread about this before, but it said it had been fixed. I use CDOSYS to send emails. I just got a reply from Comcast:
smtp;521-EHLO/HELO from sender 64.187.***.** does not map to win in DNS
(IP starred out by me)

Any ideas? It may be related to SMTP auth, but I'm going to open a different thread on that.

I'm seeing the same message. I'm using SMTP auth and the ASPEmail components.