Hi @K Gibson ,
As I know, there may be no option in server and outlook that could meet your requirement now.
Based on my research, for Microsoft 365 account, we could set the disclaimer to the message with any conditions in admin center. But for your requirement of the pop-up in Outlook, we may need to use any vba scripts with Outlook client. I found a similar thread that mentioned how to set pop up when you send the message, for your reference: How to create a custom pop up window to set sensitivity when send is clicked in Outlook 2013
Also, if you would like to post any ideas or suggestions about Outlook function, it's recommednded to post it in uservoice for Outlook, which is the official platform that Microsoft collect the suggestions on Outlook.
Hope that would be helpful to you.
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