let prefetch_should_download() check if it might be an ndn

src/dc_receive_imf.rs

@@ -2382,6 +2382,7 @@ mod tests {
         .await;
     }
 
+    // ndn = Non Delivery Notification
     async fn test_parse_ndn(
         self_addr: &str,
         foreign_addr: &str,
@@ -2415,6 +2416,14 @@ mod tests {
         let chats = Chatlist::try_load(&t.ctx, 0, None, None).await.unwrap();
         let msg_id = chats.get_msg_id(0).unwrap();
 
+        // Check that the ndn would be downloaded:
+        let headers = mailparse::parse_mail(raw_ndn).unwrap().headers;
+        assert!(
+            crate::imap::prefetch_should_download(&t.ctx, &headers, ShowEmails::Off)
+                .await
+                .unwrap()
+        );
+
         dc_receive_imf(&t.ctx, raw_ndn, "INBOX", 1, false)
             .await
             .unwrap();

src/headerdef.rs

@@ -21,6 +21,7 @@ pub enum HeaderDef {
     References,
     InReplyTo,
     Precedence,
+    ContentType,
     ChatVersion,
     ChatGroupId,
     ChatGroupName,

src/imap/mod.rs

@@ -1449,7 +1449,7 @@ async fn prefetch_is_reply_to_chat_message(
     false
 }
 
-async fn prefetch_should_download(
+pub(crate) async fn prefetch_should_download(
     context: &Context,
     headers: &[mailparse::MailHeader<'_>],
     show_emails: ShowEmails,
@@ -1457,6 +1457,16 @@ async fn prefetch_should_download(
     let is_chat_message = headers.get_header_value(HeaderDef::ChatVersion).is_some();
     let is_reply_to_chat_message = prefetch_is_reply_to_chat_message(context, &headers).await;
 
+    let maybe_ndn = if let Some(subject) = headers.get_header_value(HeaderDef::Subject) {
+        subject.to_ascii_lowercase().contains("fail")
+    } else {
+        false
+    } || if let Some(ctype) = headers.get_header_value(HeaderDef::ContentType) {
+        ctype.starts_with("multipart/report")
+    } else {
+        false
+    };
+
     // Autocrypt Setup Message should be shown even if it is from non-chat client.
     let is_autocrypt_setup_message = headers
         .get_header_value(HeaderDef::AutocryptSetupMessage)
@@ -1467,6 +1477,7 @@ async fn prefetch_should_download(
     let accepted_contact = origin.is_known();
 
     let show = is_autocrypt_setup_message
+        || maybe_ndn
         || match show_emails {
             ShowEmails::Off => is_chat_message || is_reply_to_chat_message,
             ShowEmails::AcceptedContacts => {

src/mimeparser.rs

@@ -900,7 +900,9 @@ impl MimeMessage {
         Ok(None)
     }
 
-    /// Some providers like GMX and Yahoo do not send standard NDNs (Non Delivery notifications)
+    /// Some providers like GMX and Yahoo do not send standard NDNs (Non Delivery notifications).
+    /// If you improve heuristics here you might also have to change prefetch_should_download() in imap/mod.rs.
+    /// Also you should add a test in dc_receive_imf.rs (there already are lots of test_parse_ndn_* tests).
     async fn heuristically_parse_ndn(&mut self, context: &Context) -> Option<()> {
         if self
             .get(HeaderDef::Subject)?