The Monitored Inbox option works if you are using your own MTA (e.g. Postfix) running on your server and you have configured it as “Other SMTP Server” in the dropdown on the Email Settings page. You need to set up Postfix so that it generates its own Delivery Status Notification (DSNs) as not all receiving email servers are configured to send DSNs when an address doesn’t exist - or bounces for any other reason.
After you set up Postfix to generate DSNs to your return-path email address, you can set up Mautic to parse those DSNs using the Monitored Inbox feature and automatically mark contacts as “bounced.” That being said, this feature is relatively unmaintained, as the majority of users use HTTP callback from Amazon SES, Elastic Email, Mailjet, SendGrid, SparkPost to track bounces/complaints.
Edit: Another thing you could try is going to your SES dashboard and enabling email notifications of bounces & complaints which will go to your return-path address. It’s a radio button “Yes” “No” which is located right underneath where you select the dropdowns for the SNS topic.