Carefully check that the address was spelled correctly, and try sending it again if there were any mistakes. It is also possible that a network problem caused this situation, so if you are sure the address is correct you might want to try to send it again. If the problem continues, contact your friendly system administrator. If it is a long term subscriber of yours then it is pretty surely the latter reason, if it is a new subscriber then it could be either. They have somehow broken their configuration and cannot deliver their mail internally.
Message is undeliverable.
Too many file attachments. Please send separate messages with less than 20 attachments. Usually with an explanatory note either before or after your actions of course. Hence the receiving server sends it back whence it came, to RootsWeb, then on to you. An interesting thing about this bounced message is that also demonstrates that there is a virtual domain in action here.