Start by making a file called /etc/postfix/virtual, with this content

root    receiver_email@non_local_host

Then run the below, to make a virtual lookup table.

sudo postmap /etc/postfix/virtual

To enable the virtual alias, insert the below into /etc/postfix/main.cf

virtual_alias_maps = hash:/etc/postfix/virtual

Now restart postfix, and send a mail to root

sudo /etc/init.d/postfix restart
echo "hmm" | sendmail root

This worked for me at Ubuntu Dapper and Ubuntu Edgy.