mimefactory: refactor generation of the To: field

Group all the code responsible for the generation of the To: field

src/mimefactory.rs

@@ -458,8 +458,20 @@ impl<'a> MimeFactory<'a> {
             self.from_addr.clone(),
         );
 
+        let undisclosed_recipients = match &self.loaded {
+            Loaded::Message { chat } => chat.typ == Chattype::Broadcast,
+            Loaded::Mdn { .. } => false,
+        };
+
         let mut to = Vec::new();
-        let email_to_remove = if self.msg.param.get_cmd() == SystemMessage::MemberRemovedFromGroup {
+        if undisclosed_recipients {
+            to.push(Address::new_group(
+                "hidden-recipients".to_string(),
+                Vec::new(),
+            ));
+        } else {
+            let email_to_remove =
+                if self.msg.param.get_cmd() == SystemMessage::MemberRemovedFromGroup {
                     self.msg.param.get(Param::Arg)
                 } else {
                     None
@@ -485,6 +497,7 @@ impl<'a> MimeFactory<'a> {
             if to.is_empty() {
                 to.push(from.clone());
             }
+        }
 
         headers
             .unprotected
@@ -584,20 +597,9 @@ impl<'a> MimeFactory<'a> {
             render_rfc724_mid(&rfc724_mid),
         ));
 
-        let undisclosed_recipients = match &self.loaded {
-            Loaded::Message { chat } => chat.typ == Chattype::Broadcast,
-            Loaded::Mdn { .. } => false,
-        };
-
-        if undisclosed_recipients {
-            headers
-                .unprotected
-                .push(Header::new("To".into(), "hidden-recipients: ;".to_string()));
-        } else {
         headers
             .unprotected
             .push(Header::new_with_value("To".into(), to).unwrap());
-        }
 
         headers
             .unprotected