If you use Mailgun email service then use this documentation to set up Bounce handling with FluentCRM.
First copy the Webhook URL at FluentCRM -> Settings -> SMTP/Email Sending Service Settings and Select Mailgun from the dropdown
Login to your Mailgun account and then navigate to Sending > Webhook then click on the “Add webhook” button
Select the Event type as Spam Complaints and then paste the URL that you have copied from FluentCRM for Mailgun
Then create another webhook in Mailgun for Permanent Failure
If you set up these two webhook correctly, so whenever an email mark as a complaint or Detected as a permanent Failure, Then FluentCRM will change the contact status accordingly.