diff --git a/src/headerdef.rs b/src/headerdef.rs
index 193b24b7f..879dfc1e1 100644
--- a/src/headerdef.rs
+++ b/src/headerdef.rs
@@ -12,7 +12,7 @@ pub enum HeaderDef {
     OriginalMessageId,
 
     /// Delta Chat extension for message IDs in combined MDNs
-    XAdditionalMessageIds,
+    AdditionalMessageIds,
 
     ListId,
     References,
diff --git a/src/mimefactory.rs b/src/mimefactory.rs
index ed80c34f4..b0bb52a5b 100644
--- a/src/mimefactory.rs
+++ b/src/mimefactory.rs
@@ -948,7 +948,7 @@ impl<'a, 'b> MimeFactory<'a, 'b> {
         let extension_fields = if additional_msg_ids.is_empty() {
             "".to_string()
         } else {
-            "X-Additional-Message-IDs: ".to_string()
+            "Additional-Message-IDs: ".to_string()
                 + &additional_msg_ids
                     .iter()
                     .map(|mid| render_rfc724_mid(&mid))
diff --git a/src/mimeparser.rs b/src/mimeparser.rs
index b11b0e466..1dbd8df48 100644
--- a/src/mimeparser.rs
+++ b/src/mimeparser.rs
@@ -743,7 +743,7 @@ impl<'a> MimeMessage<'a> {
                 .and_then(|v| parse_message_id(&v))
             {
                 let additional_message_ids = report_fields
-                    .get_first_value(&HeaderDef::XAdditionalMessageIds.get_headername())
+                    .get_first_value(&HeaderDef::AdditionalMessageIds.get_headername())
                     .ok()
                     .flatten()
                     .map_or_else(Vec::new, |v| {
@@ -868,7 +868,7 @@ fn update_gossip_peerstates(
 pub(crate) struct Report {
     /// Original-Message-ID header
     original_message_id: String,
-    /// X-Additional-Message-IDs
+    /// Additional-Message-IDs
     additional_message_ids: Vec<String>,
 }
 
@@ -1430,7 +1430,7 @@ Original-Recipient: rfc822;bob@example.org\n\
 Final-Recipient: rfc822;bob@example.org\n\
 Original-Message-ID: <foo@example.org>\n\
 Disposition: manual-action/MDN-sent-automatically; displayed\n\
-X-Additional-Message-IDs: <foo@example.com> <foo@example.net>\n\
+Additional-Message-IDs: <foo@example.com> <foo@example.net>\n\
 \n\
 \n\
 --kJBbU58X1xeWNHgBtTbMk80M5qnV4N--\n\