is_plugin_active isn’t available for mu-plugins to use. The codex says:
NOTE: defined in wp-admin/includes/plugin.php, so this is only available from within the admin pages, and any references to this function must be hooked to admin_init or a later action. If you want to use this function from within a template or a must-use plugin, you will need to manually require plugin.php, an example is below.
And I think there must be better ways to solve this anyway: your code will try and send you an email for every non-static HTTP request to the site, and you probably meant !is_plugin_active or is_plugin_inactive, and in any case these accept relative paths to the plugin files not absolute paths.