Normally, Postfix will reject e-mail sent to non-existent users if a catchall isn't present for the specific domain that is receiving mail. However, you can make a super catchall to catch any and all e-mail that Postfix receives for the domains in its mydestination list:
Add the following to /etc/postfix/main.cf:
luser_relay = root
local_recipient_maps =
Then reload the Postfix configuration:
# postfix reload
For more information:
http://www.postfix.org/rewrite.html#luser_relay
