All emails sent from our domain to @comcast.net addresses are being filtered out as SPAM

Hello,

We are using Exchange Server 2003 and Comcast Business Internet Service, and are unable to send any emails to Comcast recipients. I have added a PTR record, SPF record, as well as setting the exchange server to use external DNS, but that did not fix the problem. As a test I logged into my personal Comcast account settings and completely disabled SPAM filtering, and these messages instantly start coming through. This is not a viable solution though, because we can't ask every client with a Comcast email address to manually change their SPAM settings. The domain name we send from is firstfidcorp.com. Is there a way prevent emails from automatically getting marked as SPAM? I have filled out a blacklist removal request, but it says our IP is not blacklisted.  Any help would be greatly appreciated.

Thanks