diff --git a/README.md b/README.md
index 7f795f1..cf64097 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,8 @@
 
 see forum https://forum.howtoforge.com/threads/postfix-dovecot-sni-for-letsencrypt-certificates-via-cron-script-auto.86878/
 
+tutorial debian buster - https://0x85.org/postfix-dovecot-sni.html
+
 
 in postfix main.cf:
 tls_server_sni_maps = ${indexed}sni