A client of mine is experiencing the following message every time he emails someone with a Comcast account:
Subject: Delivery Status Notification (Delay)
This is an automatically generated Delivery Status Notification.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
Delivery to the following recipients has been delayed.
The message eventually times out.
I called his ISP, Megapath, and was transfered to Technical support. The tech ( Vern ) said "Ugh, please have the client forward the delay message to firstname.lastname@example.org and ask them to figure out why Comcast is blocking us AGAIN."
This went on for some time. Eventually, we found that Comcast was blocking all mail from our location on port 25. We were advised to change it to 587 on the exchange server, and everything started flowing again.