When you get mail from a Mailer-Daemon with a subject similar to "Failed Delivery," this means that a message you sent was undeliverable and has been bounced back to you. These messages are produced automatically and usually include a reason for the delivery failure.
But how do I fix it? I want these emails to be delivered!
Basically, you can't fix it. This is a system-generated error indicating that something is wrong with the address you are sending to, or a server along the way. As a check, try sending the message using some other non-Comcast account you have like Yahoo or Outlook.com or something. Does the same thing happen?