Add custom [_admin_mail] shortcode to CF7

settings.php

if ( ! function_exists( 'add_cf7_shortcode' ) ) {
	add_filter('wpcf7_special_mail_tags', 'add_cf7_shortcode', 20, 3);
	function add_cf7_shortcode($output, $name, $html) {
		$name = preg_replace('/^wpcf7\./', '_', $name);

		if ('_admin_mail' == $name) {
			if ( function_exists('get_option_tree') ) {
				$email = ot_get_option('email'); // get OT value

				$output = $email;
			}
		}

		return $output;
	}
}