code: async def apply(self, sender: str, recipient: str, mailbox: str,
                      append_msg: AppendMessage) \
              -> Tuple[Optional[str], AppendMessage]:
          
          ...
docs: Run the filter and return the mailbox where it should be appended,
          or None to discard, and the message to be appended, which is usually
          the same as ``append_msg``.
  
          Args:
              sender: The envelope sender of the message.
              recipient: The envelope recipient of the message.
              mailbox: The intended mailbox to append the message.
              append_msg: The message to be appended.
  
          raises:
              :exc:`~pymap.exceptions.AppendFailure`