[exim] Exim config option for not calling spamd for Mailman …

I am running Exim-4.42 on FreeBSD with Mailman and
Spamassassin 3.0. When a user sends out a Mailman post
to 1,000+ people, it hogs 98% CPU for quite a while.

How do I tell Exim to not call spamd for mailman posts?

This is in my Exim config:
spamd_address = 127.0.0.1 783

This is in my acl_stmp_data:
# Always add X-Spam-Score and X-Spam-Report headers, using SA system-wide settings
# (user "nobody"), no matter if over threshold or not.
warn  message = X-Spam-Score: $spam_score ($spam_bar)
      spam = noboby:true
warn  message = X-Spam-Report: $spam_report
      spam = nobody:true

I asked the Spamassassin email list first and the thought there is to
have Exim not call spamd for locally generated messages.

How do I set that up?

Thanks,
Andy