router/190_local_rcpt_domain_blacklist 

<code>
### router/200_exim4-config_primary
#################################
# This file holds the primary router, responsible for nonlocal mails

.ifdef DCconfig_internet
# configtype=internet
#
# block outgoing mail to domains listet in CONFDIF/local_rcpt_domain_blacklist

rcpt_domain_blacklist:
  debug_print = "R: rcpt_domain_blacklist for $local_part@$domain"
  driver = redirect
  no_verify_sender
  domains = CONFDIR/rcpt_domain_blacklist
  allow_fail
  data = :fail: Outgoing mail to this domain is blocked
  #data = :blackhole: # use this instead to let emails vanish
  no_more

.endif


# The "no_more" above means that all later routers are for
# domains in the local_domains list, i.e. just like Exim 3 directors.
</code>