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=