We don't use sendmail here, and the system we do use doesn't recognize
.forward.  For complete details check out man 5 delivermail, but what
you can do is add this to the .maildelivery file in your home
directory:

default	-	pipe	?	/usr/lib/sendmail ari@xerox.com

That will forward it to xerox.com if at all possible, but if sendmail
should fail it'll keep a copy at CMU.  Look up the "?", "R", and "A"
options in the man page if you want slightly different behaviour.