[Pkg-exim4-users] Exim 4 config question

[Anthony]

Hello.

In order to prevent spam, I'd like Exim 4 (I run Debian) to reject 
incoming mail
if sender's domain doesn't exist.

After some investigations, it seems that the relevants configuration 
instructions
are in /etc/exim4/conf.d/acl/30_exim4-config_check_rcpt :

# Deny unless the sender address can be verified.
#
# This is disabled by default so that DNSless systems don't break. If
# your system can do DNS lookups without delay or cost, you might want
# to enable this feature.
.ifdef CHECK_RCPT_VERIFY_SENDER
  deny
	message = Sender verification failed
  	!acl = acl_whitelist_local_deny
	!verify = sender
.endif

But where or how is set the variable CHECK_RCPT_VERIFY_SENDER
and what should I change in 30_exim4-config_check_rcpt to get what I 
want ?
I've also noticed the file /etc/exim4/update-exim4.conf.conf but I 
don't know
what to change here.

Thank you very much.