SAFE Mode with Exim:

If you don't already have one, you'll need to add a local ACL file for the RCPT ACL check.
On a split config, add something like the following to:
Then edit /etc/exim4/local_acl_check_rcpt:
deny senders = :
dnslists =
log_message = $sender_host_address listed at $dnslist_domain
message = DrMX: $dnslist_text

SAFE MODE with Postfix:

Edit /etc/postfix/
smtpd_recipient_restrictions =
check_sender_access dbm:/etc/postfix/check_drmx
Create new file: /etc/postfix/check_drmx:
<> reject_rbl_client
postmaster reject_rbl_client

Execute following commands:
postmap /etc/postfix/check_drmx
postfix reload
for changes to take effect.

NOTE: Use of any DrMX.ORG DNS Zone is by request and by contract only.