If you use Postmark 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 Postmark from the dropdown and copy the webhook URL.
Then Login to your Postmark account, Select your server, and then select your transactional message stream
Then paste the webhook URL and select Bounce and Spam Complaint. Please do not check “Include Message Content”
Then at the bottom of that page click “Save Webhook”.
If you set up these two webhook correctly, so whenever an email mark as a complaint or Detected as a bounced, Then FluentCRM will change the contact status accordingly.