feat: use UUID v4 to generate Message-IDs

This hopefully does not trigger Apple spam filter
which seems to pass thorugh Message-IDs with hyphens.
It is not confirmed, but better blend in
by mimicking existing popular email
where possible.

src/tools.rs

@@ -34,6 +34,7 @@ use num_traits::PrimInt;
 use rand::{thread_rng, Rng};
 use tokio::{fs, io};
 use url::Url;
+use uuid::Uuid;
 
 use crate::chat::{add_device_msg, add_device_msg_with_importance};
 use crate::config::Config;
@@ -275,12 +276,10 @@ async fn maybe_warn_on_outdated(context: &Context, now: i64, approx_compile_time
     }
 }
 
-/* Message-ID tools */
-
 /// Generate an unique ID.
 ///
 /// The generated ID should be short but unique:
-/// - short, because it used in Message-ID and Chat-Group-ID headers and in QR codes
+/// - short, because it used in Chat-Group-ID headers and in QR codes
 /// - unique as two IDs generated on two devices should not be the same
 ///
 /// IDs generated by this function have 144 bits of entropy
@@ -312,7 +311,15 @@ pub(crate) fn validate_id(s: &str) -> bool {
 /// - the message ID should be globally unique
 /// - do not add a counter or any private data as this leaks information unnecessarily
 pub(crate) fn create_outgoing_rfc724_mid() -> String {
-    format!("{}@localhost", create_id())
+    // We use UUID similarly to iCloud web mail client
+    // because it seems their spam filter does not like Message-IDs
+    // without hyphens.
+    //
+    // However, we use `localhost` instead of the real domain to avoid
+    // leaking the domain when resent by otherwise anonymizing
+    // From-rewriting mailing lists and forwarders.
+    let uuid = Uuid::new_v4();
+    format!("{uuid}@localhost")
 }
 
 // the returned suffix is lower-case
@@ -979,7 +986,8 @@ DKIM Results: Passed=true";
     #[test]
     fn test_create_outgoing_rfc724_mid() {
         let mid = create_outgoing_rfc724_mid();
-        assert_eq!(mid.len(), 34);
+        assert_eq!(mid.len(), 46);
+        assert!(mid.contains("-")); // It has an UUID inside.
         assert!(mid.ends_with("@localhost"));
     }