How to prohibit Copilot from processing the content of all external mails?

Yes, please leveaging auto-labeling policies would work:

1. Go to Microsoft Purview Compliance Portal
Navigate to Microsoft Purview and sign in with admin credentials.
2. Create a Sensitivity Label
- Define a label that restricts Copilot access (e.g., “External – Restricted”).
- Configure the label to block Copilot’s VIEW and EXTRACT permissions.
3. Set Up Auto-Labeling Policy
- Choose Exchange Online as the location.
- Use conditions like “Sender is external” or “Email received from outside the organization.”
- Apply the “External – Restricted” label automatically when these conditions are met.
4. Test with Simulation Mode
- Run the policy in simulation first to ensure it’s catching the right emails.
- Once verified, switch to active mode.