Comcast.net & Exch. 2K

We are a school and are using Exch 2K, our users cannot send email to the comcast.net domain. They can receive it but if they try to reply or originate a message to this domain, it hangs in a queue and eventually this comes back:

‘fitzgibon@comcast.net’ on 2/1/2003 3:28 PM
Could not deliver the message in the time limit specified. Please retry or contact your administrator.

Have poked around the knowledge base to no avail. Anyideas?