diff --git a/smtp-proxy.ini b/smtp-proxy.ini
index 980b63a..615adeb 100644
--- a/smtp-proxy.ini
+++ b/smtp-proxy.ini
@@ -18,7 +18,8 @@
 ;local_key  = smtpd.key
 
 ; Enforce encrypted connection on STARTTLS ports before
-; accepting mails from client.
+; accepting mails from client. Cannot be used with plain
+; non encrypted listeners.
 ;local_forcetls = false
 
 ; Relay all mails to this SMTP server