Add this to the file /etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport_maps
Then create the file /etc/postfix/transport_maps containing the specific address (you can try the wildcard) and the server to send the emails:
user1@example.com smtp:aspmx.l.google.com
user2@example.com smtp:aspmx.l.google.com
After, run postmap and restart postfix:
postmap /etc/postfix/transport_maps
service postfix restart