The plugin is checking the inbox and not finding anything, but you still need to make sure that the plugin runs in a timely manner. That is done using a real cron job + the webhook or the wp-cron option in the settings (without the webhook).
Please see https://usestrict.net/bbpress-notify-no-spam-reply-by-email-documentation/#to-cron-or-to-webhook
Cheers,
Vinny
