"resimta-po-32v.sys.comcast.net rejected your message..."
Most of my email messages are received and sent by my Comcast email address without incident. But for at least a couple of weeks, my Inbox receives at least one or two emails a day with a subject line such as "Undeliverable: Invoice 4173". These emails all appear to be based on actual e-mail messages either sent to my comcast email address, or sent by me to another recipient. But their subject lines always start with the word "Undeliverable". The body of the email contains apparently standard text such as this:
resimta-po-32v.sys.comcast.net rejected your message to the following email addresses:
[my email address] There's a problem with the recipient's mailbox. Please try resending the message. If the problem continues, please contact your email admin.