improve message_id

app/log.py

@@ -13,7 +13,7 @@ _log_format = '%(asctime)s - %(name)s - %(levelname)s - "%(pathname)s:%(lineno)d
 _log_formatter = logging.Formatter(_log_format)
 
 # used to keep track of an email lifecycle
-_MESSAGE_ID = None
+_MESSAGE_ID = ""
 
 
 def set_message_id(message_id):

email_handler.py

@@ -1788,7 +1788,8 @@ class MailHandler:
         )
 
         # generate a different message_id to keep track of an email lifecycle
-        set_message_id(str(uuid.uuid4()))
+        message_id = str(uuid.uuid4())
+        set_message_id(message_id)
 
         app = new_app()
         with app.app_context():