Howto send a bounce e-mail message?

I've setup one of my domains to catchAll-Email.
Now I want to send some bounce e-mails for some e-mail addresses from this domain.

Is a bounced email only a reply to the sender with as subject: "Undelivered Mail Returned to Sender", or do I also need to send somekind of "550" code with it?

I've had a look at Falko's "How To Configure Custom Postfix Bounce Messages", but this will not work if I have set the catchAll-Email option.

 

Kind of found a way of doing this (in PHP)
mail($To,$Subject,$Msg,$headers, '-f'.$returnaddress);