You're being blocked because any mail leaving Comcast's IP spaces is expected to come from Comcast's mailservers only.
Configure your mailserver with a "smarthost" option, have it deliver using Authenticated SMTP (with your Comcast account's username and password hardcoded, yes) over SSL on 465, or if you can't do SSL, use 587.
Source: Am currently running Postfix on Comcast successfully delivering to Yahoo Mail with no spamfolder problem via this method. (Am using SPF, no DomainKeys yet.)
More from Comcast on this: http://corporate.comcast.com/c...