Do not send read receipts for system messages

src/job.rs

@@ -671,7 +671,18 @@ impl Job {
                 // we want to send out an MDN anyway
                 // The job will not be retried so locally
                 // there is no risk of double-sending MDNs.
+                //
+                // Read receipts for system messages are never
+                // sent. These messages have no place to display
+                // received read receipt anyway.  And since their text
+                // is locally generated, quoting them is dangerous as
+                // it may contain contact names. E.g., for original
+                // message "Group left by me", a read receipt will
+                // quote "Group left by <name>", and the name can be a
+                // display name stored in address book rather than
+                // the name sent in the From field by the user.
                 if msg.param.get_bool(Param::WantsMdn).unwrap_or_default()
+                    && !msg.is_system_message()
                     && context.get_config_bool(Config::MdnsEnabled).await
                 {
                     if let Err(err) = send_mdn(context, &msg).await {

src/message.rs

@@ -600,6 +600,11 @@ impl Message {
             || cmd != SystemMessage::Unknown && cmd != SystemMessage::AutocryptSetupMessage
     }
 
+    pub fn is_system_message(&self) -> bool {
+        let cmd = self.param.get_cmd();
+        cmd != SystemMessage::Unknown
+    }
+
     /// Whether the message is still being created.
     ///
     /// Messages with attachments might be created before the

src/mimefactory.rs

@@ -113,12 +113,7 @@ impl<'a, 'b> MimeFactory<'a, 'b> {
                 )
                 .await?;
 
-            let command = msg.param.get_cmd();
+            if !msg.is_system_message() && context.get_config_bool(Config::MdnsEnabled).await {
-
-            if command != SystemMessage::AutocryptSetupMessage
-                && command != SystemMessage::SecurejoinMessage
-                && context.get_config_bool(Config::MdnsEnabled).await
-            {
                 req_mdn = true;
             }
         }