diff --git a/README.md b/README.md
index 08c2bac..f8dbce8 100644
--- a/README.md
+++ b/README.md
@@ -9,3 +9,4 @@ Information about how to configure email servers, services, etc.
 - https://dnssec-debugger.verisignlabs.com/
 - https://www.experte.de/dns-check/dnssec
 - https://ssl-tools.net/
+- https://www.nospamproxy.de/wp-content/uploads/Praxisleitfaden-DKIM-SPF-DMARC-DANE-1.pdf