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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _raise_error_if_disconnected(self) -> None: """ See if we're still connected, and if not, raise ``SMTPServerDisconnected``. """

        if (
            self.transport is None
            or self.protocol is None
            or self.transport.is_closing()
        ):
            self.close()
            raise SMTPServerDisconnected("Disconnected from SMTP server")