diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 612b068004..1baf1ea1d3 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -966,6 +966,7 @@ public class MessageHelper {
         ensureEnvelope();
 
         // Outlook outbox -> sent
+        //   x-microsoft-original-message-id
         String header = imessage.getHeader(HEADER_CORRELATION_ID, null);
         if (header == null)
             header = imessage.getHeader("Message-ID", null);