feature req: automatic "no such user"

[urgrue <urgrue tumsan fi>]

i think it would be a nice feature in any mail program, if you could just 
hit a button, and the mail client would send an automatic message back to 
the sender that looks just like a normal sendmail SMTP error 550 (no such 
user).
this won't solve the world's spam problem, but i use deadaddress on my 
windows desktop (which does exactly this), and it does seem like i get less 
spam now than a while ago (of course this could also be all 
psychological..."my car runs better after i wash it"...)
i assume it would be extremely simple to implement...?
fred