Remote server dropping connection with Comcast.net in Exchange

I'm running Exchange 2003 on Server 2003. I've created email distribution lists via mail-enabled public folders. The problem I'm having is that when I send emails to the distribution list, comcast.net is the only one that doesn't recieve emails from my list. In Exchange System Manager, it tells me "the remote server dropped the connection" and in Event Viewer, I get the following error for every comcast.net recipient:

"This is an SMTP protocol error log for virtual server ID 1, connection #900. The remote host "63.240.76.26", responded to the SMTP command "rcpt" with "452 too many recipients with no from address, send more in a second session  ". The full command sent was "RCPT TO:<xxx@comcast.net> NOTIFY=NEVER  ".  This will probably cause the connection to fail."

How can I resolve this? Comcast is the only ISP i'm having this issue with.