Only for O365 email boxes and Exchange, you can change the subject of the email server after process, for example append a text to tell this email has been processed.
Add the action
2. Select the action 'Export emails in a CSV database' in the list
3. Fill the settings, very simple
- Prepend text: Add text before the existing subject
- Append text: Add text after the existing subject
- Search text / And replace with text: Enter the text to search and replace it. It can be a simple text, regex not authorized.