fix: do not set WantsMdn param for outgoing messages

This bug sometimes results in sending read receipts to self in
multi-device setups.

It happens consistently in a setup where the first device is
configured to move messages to DeltaChat folder and the second device
is not. When both devices receive BCC-self message simultaneously, the
first device moves the message to DeltaChat folder, while the second
device tries to mark the message as seen in the Inbox. Regardless of
whether the second device marks the message as seen successfully or
fails because the message is already moved by the first device,
`Job.markseen_msg_on_imap()` sends the read receipt to the From
address.

src/mimeparser.rs

@@ -276,7 +276,7 @@ impl MimeMessage {
         parser.maybe_remove_bad_parts();
         parser.maybe_remove_inline_mailinglist_footer();
         parser.heuristically_parse_ndn(context).await;
-        parser.parse_headers(context).await;
+        parser.parse_headers(context).await?;
 
         if warn_empty_signature && parser.signatures.is_empty() {
             for part in parser.parts.iter_mut() {
@@ -419,7 +419,7 @@ impl MimeMessage {
         }
     }
 
-    async fn parse_headers(&mut self, context: &Context) {
+    async fn parse_headers(&mut self, context: &Context) -> Result<()> {
         self.parse_system_message_headers(context);
         self.parse_avatar_headers(context).await;
         self.parse_videochat_headers();
@@ -464,15 +464,20 @@ impl MimeMessage {
         if !self.decrypting_failed && !self.parts.is_empty() {
             if let Some(ref dn_to) = self.chat_disposition_notification_to {
                 if let Some(from) = self.from.get(0) {
-                    if from.addr.to_lowercase() == dn_to.addr.to_lowercase() {
-                        if let Some(part) = self.parts.last_mut() {
-                            part.param.set_int(Param::WantsMdn, 1);
+                    // Check that the message is not outgoing.
+                    if !context.is_self_addr(&from.addr).await? {
+                        if from.addr.to_lowercase() == dn_to.addr.to_lowercase() {
+                            if let Some(part) = self.parts.last_mut() {
+                                part.param.set_int(Param::WantsMdn, 1);
+                            }
+                        } else {
+                            warn!(
+                                context,
+                                "{} requested a read receipt to {}, ignoring",
+                                from.addr,
+                                dn_to.addr
+                            );
                         }
-                    } else {
-                        warn!(
-                            context,
-                            "{} requested a read receipt to {}, ignoring", from.addr, dn_to.addr
-                        );
                     }
                 }
             }
@@ -502,6 +507,8 @@ impl MimeMessage {
                 part.param.set(Param::Bot, "1");
             }
         }
+
+        Ok(())
     }
 
     async fn avatar_action_from_header(
@@ -2954,4 +2961,36 @@ Some reply
 
         Ok(())
     }
+
+    // Test that WantsMdn parameter is not set on outgoing messages.
+    #[async_std::test]
+    async fn test_outgoing_wants_mdn() -> Result<()> {
+        let alice = TestContext::new_alice().await;
+        let bob = TestContext::new_bob().await;
+
+        let raw = br###"Date: Thu, 28 Jan 2021 00:26:57 +0000
+Chat-Version: 1.0\n\
+Message-ID: <foobarbaz@example.org>
+To: Bob <bob@example.org>
+From: Alice <alice@example.com>
+Subject: subject
+Chat-Disposition-Notification-To: alice@example.com
+
+Message.
+"###;
+
+        // Bob receives message.
+        dc_receive_imf(&bob, raw, "INBOX", 1, false).await?;
+        let msg = bob.get_last_msg().await;
+        // Message is incoming.
+        assert!(msg.param.get_bool(Param::WantsMdn).unwrap());
+
+        // Alice receives copy-to-self.
+        dc_receive_imf(&alice, raw, "INBOX", 1, false).await?;
+        let msg = alice.get_last_msg().await;
+        // Message is outgoing, don't send read receipt to self.
+        assert!(msg.param.get_bool(Param::WantsMdn).is_none());
+
+        Ok(())
+    }
 }