question on virtusertable - sendmail

[David Banning]

> @domain.com		bounce
> 
> You can define "bounce" in /etc/mail/aliases, or just leave it undefined 
> (since it will probably bounce anyway).

I am looking at my /etc/mail/aliases

how would I define bounce?

I know I could simple send it to /dev/null, but I want it to bounce 
back to the sender with a "no such user" error.