feat: send to hidden recipients if the To field is empty

Instead of adding self to the To field,
create an empty group in the To field.
It is important for group consistency
algorithm that if Alice removes Bob
from a group consisting only of the Alice and Bob,
she sends "member removed" message
with empty group in the To field
rather than self address in the To field.

Author: link2xt

src/mimefactory.rs

@@ -494,12 +494,7 @@ impl MimeFactory {
         };
 
         let mut to = Vec::new();
-        if undisclosed_recipients {
-            to.push(Address::new_group(
-                "hidden-recipients".to_string(),
-                Vec::new(),
-            ));
-        } else {
+        if !undisclosed_recipients {
             let email_to_remove = match &self.loaded {
                 Loaded::Message { msg, .. } => {
                     if msg.param.get_cmd() == SystemMessage::MemberRemovedFromGroup {
@@ -524,10 +519,13 @@ impl MimeFactory {
                     to.push(new_address_with_name(name, addr.clone()));
                 }
             }
+        }
 
-            if to.is_empty() {
-                to.push(from.clone());
-            }
+        if to.is_empty() {
+            to.push(Address::new_group(
+                "hidden-recipients".to_string(),
+                Vec::new(),
+            ));
         }
 
         // Start with Internet Message Format headers in the order of the standard example