Route unknown user to new domain

[luis.dominguez]

Hello. I'm using postfix + dovecot for my example.com domain, but i want 
that any missing local users change the mail to @net.example.com and go 
to my relay host. I was configured luser in postfix but when I send a 
mail to a unknown user is not working. I'm using LDA with dovecotEnviado 
desde mi smartphone Samsung Galaxy.