diff --git a/core/postfix/conf/master.cf b/core/postfix/conf/master.cf
index cbcc5e56..15fd62dc 100644
--- a/core/postfix/conf/master.cf
+++ b/core/postfix/conf/master.cf
@@ -8,6 +8,7 @@ smtp      inet  n       -       n       -       -       smtpd
 10025     inet  n       -       n       -       -       smtpd
   -o smtpd_sasl_auth_enable=yes
   -o smtpd_recipient_restrictions=reject_unlisted_sender,reject_authenticated_sender_login_mismatch,permit
+  -o smtpd_reject_unlisted_recipient={% if REJECT_UNLISTED_RECIPIENT %}{{ REJECT_UNLISTED_RECIPIENT }}{% else %}no{% endif %}
   -o cleanup_service_name=outclean
 outclean  unix n       -       n       -       0       cleanup
   -o header_checks=pcre:/etc/postfix/outclean_header_filter.cf
diff --git a/docs/compose/.env b/docs/compose/.env
index 9477448a..721aaf22 100644
--- a/docs/compose/.env
+++ b/docs/compose/.env
@@ -132,3 +132,6 @@ REAL_IP_HEADER=
 
 # IPs for nginx set_real_ip_from (CIDR list separated by commas)
 REAL_IP_FROM=
+
+# choose wether mailu bounces (no) or rejects (yes) mail when recipient is unknown (value: yes, no)
+REJECT_UNLISTED_RECIPIENT=