diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 04be2b9034..46a106be97 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -376,6 +376,7 @@ public class MessageHelper {
         }
 
         // Sensitivity
+        // https://datatracker.ietf.org/doc/html/rfc4021#section-2.1.55
         if (EntityMessage.SENSITIVITY_PERSONAL.equals(message.sensitivity))
             imessage.addHeader("Sensitivity", "Personal");
         else if (EntityMessage.SENSITIVITY_PRIVATE.equals(message.sensitivity))