diff --git a/README.md b/README.md
index b631ce8..7014ea3 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,8 @@ TLS certs are obtained via lets encrypt
 
 - https://www.appmaildev.com/en/dkim
 
+- https://sendtestemail.com/
+
 ## Extremely useful documentation
 
 - http://www.postfix.org/documentation.html