diff --git a/src/mimefactory.rs b/src/mimefactory.rs
index 27c1b5a1cc..cd9ddc6bc2 100644
--- a/src/mimefactory.rs
+++ b/src/mimefactory.rs
@@ -72,6 +72,13 @@ pub struct MimeFactory {
     /// Vector of pairs of past group member name and addresses.
     past_members: Vec<(String, String)>,
 
+    /// Timestamps of the members in the same order as in the `recipients`
+    /// followed by `past_members`.
+    ///
+    /// If this is not empty, its length
+    /// should be the sum of `recipients` and `past_members` length.
+    member_timestamps: Vec<i64>,
+
     timestamp: i64,
     loaded: Loaded,
     in_reply_to: String,
@@ -150,6 +157,7 @@ impl MimeFactory {
 
         let mut recipients = Vec::with_capacity(5);
         let mut past_members = Vec::new();
+        let mut member_timestamps = Vec::new();
         let mut recipient_ids = HashSet::new();
         let mut req_mdn = false;
 
@@ -190,6 +198,7 @@ impl MimeFactory {
                             if add_timestamp >= remove_timestamp {
                                 if !recipients_contain_addr(&recipients, &addr) {
                                     recipients.push((name, addr));
+                                    member_timestamps.push(add_timestamp);
                                 }
                                 recipient_ids.insert(id);
                             } else {
@@ -197,6 +206,7 @@ impl MimeFactory {
                                 // member is not actually part of the group.
                                 if !recipients_contain_addr(&past_members, &addr) {
                                     past_members.push((name, addr));
+                                    member_timestamps.push(remove_timestamp);
                                 }
                             }
                         }
@@ -204,6 +214,7 @@ impl MimeFactory {
                     },
                 )
                 .await?;
+
             let recipient_ids: Vec<_> = recipient_ids.into_iter().collect();
             ContactId::scaleup_origin(context, &recipient_ids, Origin::OutgoingTo).await?;
 
@@ -240,6 +251,10 @@ impl MimeFactory {
         };
         let attach_selfavatar = Self::should_attach_selfavatar(context, &msg).await;
 
+        debug_assert!(
+            member_timestamps.is_empty()
+                || recipients.len() + past_members.len() == member_timestamps.len()
+        );
         let factory = MimeFactory {
             from_addr,
             from_displayname,
@@ -247,6 +262,7 @@ impl MimeFactory {
             selfstatus,
             recipients,
             past_members,
+            member_timestamps,
             timestamp: msg.timestamp_sort,
             loaded: Loaded::Message { msg, chat },
             in_reply_to,
@@ -276,6 +292,7 @@ impl MimeFactory {
             selfstatus: "".to_string(),
             recipients: vec![("".to_string(), contact.get_addr().to_string())],
             past_members: vec![],
+            member_timestamps: vec![],
             timestamp,
             loaded: Loaded::Mdn {
                 rfc724_mid,
@@ -565,7 +582,22 @@ impl MimeFactory {
         headers.push(Header::new_with_value("To".into(), to.clone()).unwrap());
         if !past_members.is_empty() {
             headers.push(
-                Header::new_with_value("Chat-Group-Past-Members".into(), past_members.clone()).unwrap(),
+                Header::new_with_value("Chat-Group-Past-Members".into(), past_members.clone())
+                    .unwrap(),
+            );
+        }
+
+        if !undisclosed_recipients && !self.member_timestamps.is_empty() {
+            headers.push(
+                Header::new_with_value(
+                    "Chat-Group-Member-Timestamps".into(),
+                    self.member_timestamps
+                        .iter()
+                        .map(|ts| ts.to_string())
+                        .collect::<Vec<String>>()
+                        .join(" "),
+                )
+                .unwrap(),
             );
         }