Improve drafts detection

systemli.org seems not to include Received: header in messages sent to
self, including BCC-self and messages sent to Saved message chat. This
results in all these messages not being displayed on other devices in
multi-device setting. To work around this problem we restrict draft
detection based on Received: header to classical mails.

Delta Chat already implements proper draft detection based on the
\Draft message flag and \Drafts folder flag. However, Thunderbird does
not set \Draft flag when message is stored as "Template". To make
draft detection for Thunderbird more robust, in case Received: header
check is removed altogether later, we check for X-Mozilla-Draft-Info
header which Thunderbird sets both for "Drafts" and "Templates".

src/dc_receive_imf.rs

@@ -685,11 +685,26 @@ async fn add_parts(
             }
         }
 
-        if !context.is_sentbox(server_folder).await?
+        // If the message is outgoing AND there is no Received header AND it's not in the sentbox,
+        // then ignore the email.
+        //
+        // We only apply this heuristic to classical emails, as it is not reliable (some servers
+        // such as systemli.org in June 2021 remove their own Received headers on incoming mails)
+        // and we know Delta Chat never stores drafts on IMAP servers.
+        let is_draft = !context.is_sentbox(server_folder).await?
             && mime_parser.get(HeaderDef::Received).is_none()
-        {
+            && mime_parser.get(HeaderDef::ChatVersion).is_none();
+
+        // Mozilla Thunderbird does not set \Draft flag on "Templates", but sets
+        // X-Mozilla-Draft-Info header, which can be used to detect both drafts and templates
+        // created by Thunderbird.
+        //
+        // This check is not necessary now, but may become useful if the `Received:` header check
+        // is removed completely later.
+        let is_draft = is_draft || mime_parser.get(HeaderDef::XMozillaDraftInfo).is_some();
+
+        if is_draft {
             // Most mailboxes have a "Drafts" folder where constantly new emails appear but we don't actually want to show them
-            // So: If it's outgoing AND there is no Received header AND it's not in the sentbox, then ignore the email.
             info!(context, "Email is probably just a draft (TRASH)");
             *chat_id = DC_CHAT_ID_TRASH;
             allow_creation = false;

src/headerdef.rs

@@ -25,6 +25,12 @@ pub enum HeaderDef {
     /// we need to check that header as well.
     XMicrosoftOriginalMessageId,
 
+    /// Thunderbird header used to store Draft information.
+    ///
+    /// Thunderbird 78.11.0 does not set \Draft flag on messages saved as "Template", but sets this
+    /// header, so it can be used to ignore such messages.
+    XMozillaDraftInfo,
+
     ListId,
     References,
     InReplyTo,