@property<EOL><INDENT>def subscription_substitution_tag(self):<DEDENT>
return self._subscription_substitution_tag<EOL>
A tag that will be replaced with the unsubscribe URL. for example:
           [unsubscribe_url]. If this parameter is used, it will override both
           the text and html parameters. The URL of the link will be placed at
           the substitution tag's location, with no additional formatting.

        :rtype: string