I just sent an email from one of my emails through the comcast mail server and I got this:
Reporting-MTA: dns; omta01.westchester.pa.hmc1.comcast.net [184.108.40.206]
Received-From-MTA: dns; xxxxxx[220.127.116.11]
Arrival-Date: Fri, 10 May 2013 18:47:41 +0000
Final-recipient: rfc822; email@example.com
Diagnostic-Code: smtp; 550 SC-002 (SNT0-MC4-F20) Unfortunately, messages from 18.104.22.168 weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
Last-attempt-Date: Fri, 10 May 2013 18:47:41 +0000