diff --git a/README.md b/README.md
index acda3c0..79fcedc 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,8 @@ TLS certs are obtained via lets encrypt
 
 - https://sendtestemail.com/
 
+- https://mxtoolbox.com
+
 ## Extremely useful documentation
 
 - https://www.postfix.org/documentation.html