This is a VBA script to present a confirmation dialog to the user if they click on Reply All in Outlook. The idea is to discourage accidentally replying to all.
I have tested this on Outlook 2016. Other supported versions of Outlook that are allowed to run VBA should also work.
If Outlook is configured to have a preview pane visible, clicking on Reply All from the preview may skip the warning dialog. A work-around would be to disable the reading pane, or remove the Reply All button from the preview pane.
Because this code functions as a macro, you will need to allow Outlook to run macros, which is not ideal. You can help make this a little more secure by self-signing the code and restricting Outlook to only running signed code.
Enable macros (or self-sign it and enable signed macros)
Enable developer tools in Outlook. Navigate to Project1->Microsoft Outlook Objects->ThisOutlookSession. Copy and paste the text from replyall.txt into ThisOutlookSession.