>Recipient address rejected: undeliverable address: unknown user //

Hmm, it's okay.

It's a person masquerading as a computer masquerading as a person. This doesn't really help IMO. Why can't it be

"Hi, this is the Yahoo! email support team. You sent a message to someone but when we tried to pass it on the email address didn't work so we can't send the email for you. Here's some things to try ... technical details and the text of the email you sent are below [or attached]."

The response could also include info like that the domain is live.