feat: include 3 recent Message-IDs in References header

Do not include oldest reference, because chat members
which have been added later and have not seen the first message
do not have referenced message in the database.

Instead, include up to 3 recent Message-IDs.

src/receive_imf/tests.rs

@@ -4351,3 +4351,45 @@ async fn test_multiline_iso_8859_1_subject() -> Result<()> {
     );
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_references() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+    alice.set_config_bool(Config::BccSelf, true).await?;
+
+    let alice_chat_id = create_group_chat(alice, ProtectionStatus::Unprotected, "Group").await?;
+    let _sent = alice
+        .send_text(alice_chat_id, "Hi! I created a group.")
+        .await;
+
+    let alice_bob_contact_id = Contact::create(alice, "Bob", "bob@example.net").await?;
+    add_contact_to_chat(alice, alice_chat_id, alice_bob_contact_id).await?;
+    let sent = alice.pop_sent_msg().await;
+    let bob_received_msg = bob.recv_msg(&sent).await;
+    let bob_chat_id = bob_received_msg.chat_id;
+
+    // Alice sends another three messages, but two of them are lost.
+    let _sent = alice.send_text(alice_chat_id, "Second message").await;
+    let _sent = alice.send_text(alice_chat_id, "Third message").await;
+
+    // Message can still be assigned based on the `References` header.
+    let sent = alice.send_text(alice_chat_id, "Fourth message").await;
+    let bob_parsed_message = bob.parse_msg(&sent).await;
+    let bob_parent_message = get_parent_message(bob, &bob_parsed_message).await?.unwrap();
+    assert_eq!(bob_chat_id, bob_parent_message.chat_id);
+
+    // If more messages are lost, message cannot be assigned to the correct chat
+    // without `Chat-Group-ID` header, e.g. if the message is partially downloaded.
+    let sent = alice.send_text(alice_chat_id, "Fifth message").await;
+    let bob_parsed_message = bob.parse_msg(&sent).await;
+    let bob_parent_message = get_parent_message(bob, &bob_parsed_message).await?;
+    assert!(bob_parent_message.is_none());
+
+    // When the message is received, it is assigned correctly because of `Chat-Group-ID` header.
+    let bob_received_msg = bob.recv_msg(&sent).await;
+    assert_eq!(bob_chat_id, bob_received_msg.chat_id);
+
+    Ok(())
+}