diff --git a/install_postfix_advanced.sh b/install_postfix_advanced.sh
index 0b9b765..c47d936 100755
--- a/install_postfix_advanced.sh
+++ b/install_postfix_advanced.sh
@@ -2548,7 +2548,12 @@ smtpd_client_restrictions =
    reject_rhsbl_helo dbl.spamhaus.org=127.0.1.[2..99],
    reject_rhsbl_reverse_client dbl.spamhaus.org=127.0.1.[2..99],
    reject_rhsbl_sender dbl.spamhaus.org=127.0.1.[2..99],
-   reject_rbl_client zen.spamhaus.org=127.0.1.[2..99],
+   #  Only reject on real listings. ZEN answers with 127.0.0.2 - 127.0.0.11,
+   #  DBL with 127.0.1.2 - 127.0.1.99. Do NOT use these lists without a range:
+   #  If Spamhaus blocks our queries, it answers every query with an error code
+   #  like 127.255.255.254. Without a range, all mail would then be rejected.
+   #
+   reject_rbl_client zen.spamhaus.org=127.0.0.[2..11],
 
    # Greylisting check
    #