You never mentioned the reason for bouncing. Could be Hotmail, Yahoo, user input error, too big a file attachment, possibly local ISP issue, etc.. I believe ownership of the DNS addresses was handed over Oct 1.
Check the email address, used before or new, other email issues or just Yahoo accounts, try a "reply"back to that user on their email, have a friend with gmail try, take 2 drinks and try in morning, etc.
Email (like postal mail) is not perfect and some mail is rejected in error. Seems to never happen on spam emails, however, for normal users not common but happens.