fix: Never send empty To: header (#6663)

fix #6662 by adding "hidden-recipients:" if To: header would be empty

src/mimefactory.rs

@@ -607,10 +607,7 @@ impl MimeFactory {
                 || to.len() + past_members.len() == self.member_timestamps.len()
         );
         if to.is_empty() {
-            to.push(Address::new_group(
-                Some("hidden-recipients".to_string()),
-                Vec::new(),
-            ));
+            to.push(hidden_recipients());
         }
 
         // Start with Internet Message Format headers in the order of the standard example
@@ -888,21 +885,23 @@ impl MimeFactory {
             } else if header_name == "to" {
                 protected_headers.push(header.clone());
                 if is_encrypted {
+                    let mut to_without_names = to
+                        .clone()
+                        .into_iter()
+                        .filter_map(|header| match header {
+                            Address::Address(mb) => Some(Address::Address(EmailAddress {
+                                name: None,
+                                email: mb.email,
+                            })),
+                            _ => None,
+                        })
+                        .collect::<Vec<_>>();
+                    if to_without_names.is_empty() {
+                        to_without_names.push(hidden_recipients());
+                    }
                     unprotected_headers.push((
                         original_header_name,
-                        Address::new_list(
-                            to.clone()
-                                .into_iter()
-                                .filter_map(|header| match header {
-                                    Address::Address(mb) => Some(Address::Address(EmailAddress {
-                                        name: None,
-                                        email: mb.email,
-                                    })),
-                                    _ => None,
-                                })
-                                .collect::<Vec<_>>(),
-                        )
-                        .into(),
+                        Address::new_list(to_without_names).into(),
                     ));
                 } else {
                     unprotected_headers.push(header.clone());
@@ -1633,6 +1632,10 @@ impl MimeFactory {
     }
 }
 
+fn hidden_recipients() -> Address<'static> {
+    Address::new_group(Some("hidden-recipients".to_string()), Vec::new())
+}
+
 async fn build_body_file(context: &Context, msg: &Message) -> Result<MimePart<'static>> {
     let file_name = msg.get_filename().context("msg has no file")?;
     let suffix = Path::new(&file_name)

src/mimefactory/mimefactory_tests.rs

@@ -898,3 +898,23 @@ async fn test_dont_remove_self() -> Result<()> {
 
     Ok(())
 }
+
+/// Regression test: mimefactory should never create an empty to header,
+/// also not if the Selftalk parameter is missing
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_no_empty_to_header() -> Result<()> {
+    let alice = &TestContext::new_alice().await;
+    let mut self_chat = alice.get_self_chat().await;
+    self_chat.param.remove(Param::Selftalk);
+    self_chat.update_param(alice).await?;
+
+    let payload = alice.send_text(self_chat.id, "Hi").await.payload;
+    assert!(
+        // It would be equally fine if the payload contained `To: alice@example.org` or similar,
+        // as long as it's a valid header
+        payload.contains("To: \"hidden-recipients\": ;"),
+        "Payload doesn't contain correct To: header: {payload}"
+    );
+
+    Ok(())
+}