Hi there!
I am a software developer with experience working with Outlook Addins.
This project can be done as an Outlook VSTO Addin where it can have a special tab in the ribbon to set the words to search in the emails.
Once the users presses the Send button, it will show a popup window prompting the user to select an option (GDPR or Non-GDPR), once the option is selected, the proper action will be executed.
I still have some questions about what you need, specifically when receiving emails, it'd be great if we can chat to talk about the details of this project.
My bid amount comes from an estimation of 8 hrs of work @20USD.
Looking forward to hearing from you.