Mmmm, to my mind "recipient" would definitely indicate a problem with the receiving email address, not yours. I'm trying to think if I've ever had a response like that from my own mailing list, and I think I've had something *very* similar.
Is this the first time you've had it from this person? If it is, I'd leave it on your list. If not, I'd probably delete them. I think date-bound e-mail addresses (e.g. Hotmail, where you need to check your Inbox every 60 days or such to keep the account) can return this type of error message to senders when the recipient is currently inactive.
Um... OK, I'm guessing rather than knowing.
