Postfix and Comcast problem

I have a Fedora Core 14 server running with postfix as my mail program. When mail is sent to addresses for Comcast those recipients never get the messages. All others like yahoo, hotmail, gmail and more have no issues. Only Comcast. Has any one else run into this and is there a fix.

Thanks

What it is in your mail log?

I looked in what I thought was my mail logs but they seem to be blank. Mybe I am looking in the wrong directory.