Q: Is there a way to set up a rule or otherwise have Outlook provide a warning if an email was sent from someone outside your organization?
A: Thanks for asking. It is a rule but is a rule that you apply on the server. If you use the hosted Microsoft Exchange server with Office 365 here are the instructions to follow, which have to be implemented by someone with administrative rights. If you have a physical (on-premise) Microsoft Exchange server then the instructions are similar. For G-Suite users check this video.