[exim] problem(?) - SpamAssassin checks outgoing mail

Hi,

I'm using SpamAssassin 3.0.4 with my Exim 4.54 server, but SpamAssassin is
checking and adding it's headers even with outgoing mail. My logic tells me
that's not correct, but I don't know how to change this behaviour. Below is
the relevant exim.conf setup. Can anybody help me with this, please?

begin acl

acl_check_data:

  warn   spam    = nobody
         message = X-is-spam: over spam threshold\n\
                   X-Spam-Status: Yes
  warn   message = X-Spam_score: $spam_score\n\
                   X-Spam_score_int: $spam_score_int\n\
                   X-Spam_bar: $spam_bar\n\
                   X-Spam_report: $spam_report
  accept

Thanks much,
Robert