A copy of my message bounced back into my inbox. What's going on?
Your subscriber has an ISP with a broken mail system.
If a message is not deliverable, the ISP should return it to the sending computer (IE mail-list.com). Instead, they are returning it to you, the address in the From: line.
Mail-list.com computers did not see this bounced message. You may continue to receive this bounce until you remove that subscriber's email address from your list. See this post for step-by-step directions to remove an address.