diff --git a/docker/config/kopano/packages/config.sh b/docker/config/kopano/packages/config.sh
index a0c8839..872503d 100755
--- a/docker/config/kopano/packages/config.sh
+++ b/docker/config/kopano/packages/config.sh
@@ -17,6 +17,7 @@ postconf -e virtual_mailbox_maps=mysql:/etc/postfix/mysql-aliases.cf
 postconf -e virtual_alias_maps=mysql:/etc/postfix/mysql-groups.cf
 postconf -e virtual_transport=lmtp:127.0.0.1:2003
 postconf -e virtual_mailbox_domains=$MYDOMAIN
+postconf -e smtpd_relay_restrictions=defer_unauth_destination
 
 # get aliases from database.
 cat <<EOF > /etc/postfix/mysql-aliases.cf