[exim] Bounce message => Spam

Hi,

I contact you to have some help on exim4 regarding bounce messages.

I wrote in the acl_check_rcpt acl those lines :
accept  domains       = +local_domains
          endpass
          message       = unknown user / l'utilisateur n'existe pas
          verify        = recipient

and then i wrote : bounce_return_message = false
because I don't wan't original messages to be send : before, when
spammer send me spam messages, exim replied with a bounce message
including the spam message, so I was considered like a spammer by
spamcop !

But When I put those options, gmail see me like a spammer !
I really don't understand.
How can I do, to reply when the user doesn't exist without becomming a spammer ?

thanks for your help ;-)
JPB