If someone knew - they might not tell you - you could be another Mark Chapman.

If this is genuine - send them to someone who is accessible like Joan Baez - enclose the postage and ask her to forward them - or you could try his book publisher.

But its a bit sad. Haven't you got any friends who would like a Christmas card? if he wanted Christmas cards, Bob wouldn't employ people to send them back to you. Perhaps its Bob himself who sends them back.