fix(message-threading): Take non-standard but conventional subject prefixes into account

Author: louis-lau

Diff for: lib/message-handler.js

@@ -1456,12 +1456,13 @@ class MessageHandler {
         options = options || {};
         subject = subject.replace(/\s+/g, ' ').trim();
 
+        // `Re: [EXTERNAL] Re: Fwd: Example subject (fwd)` becomes `Example subject`
         if (options.removePrefix) {
             let match = true;
             while (match) {
                 match = false;
                 subject = subject
-                    .replace(/^(re|fwd?)\s*:|\s*\(fwd\)\s*$/gi, () => {
+                    .replace(/^(re|fwd?)\s*:|^\[.+?\](?=\s.+)|\s*\(fwd\)\s*$/gi, () => {
                         match = true;
                         return '';
                     })