diff --git a/core/postfix/conf/main.cf b/core/postfix/conf/main.cf
index 7a2d0fe6..37fa70d0 100644
--- a/core/postfix/conf/main.cf
+++ b/core/postfix/conf/main.cf
@@ -121,7 +121,7 @@ smtpd_relay_restrictions =
 
 unverified_recipient_reject_reason = Address lookup failure
 
-smtpd_authorized_xclient_hosts={{ SUBNET}},{{ SUBNET6 }}
+smtpd_authorized_xclient_hosts={{ SUBNET}}{% if SUBNET6 %},[{{ SUBNET6 }}]{% endif %}
 
 ###############
 # Milter