diff --git a/postfix/main.cf b/postfix/main.cf
index e47bc0d..a50120e 100644
--- a/postfix/main.cf
+++ b/postfix/main.cf
@@ -43,7 +43,7 @@ smtpd_sasl_path = /var/spool/postfix/private/auth
 smtpd_sasl_security_options = noanonymous
 smtpd_sasl_local_domain = $myhostname
 smtpd_relay_restrictions = permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination
-smtpd_sender_restrictions = permit_sasl_authenticated,permit_mynetworks
+smtpd_sender_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unknown_sender_domain
 
 non_smtpd_milters=inet:127.0.0.1:8891
 smtpd_milters=inet:127.0.0.1:8891