A "401 Unauthorized" error typically indicates an issue with authentication or permissions. Since the Client ID and Client Secret have been successfully used in other REST calls, the most likely cause is insufficient permissions for sending emails.

Check Send Permissions: Ensure that the appropriate permissions for sending emails have been granted to the Client ID and Client Secret in the Installed Packages. Without these permissions, the API call will fail with a 401 error.

Salesforce Installed Packages and Permissions