Outlook generates a spoofed message when you forward an external meeting request, which is probably going to be treated as spam if the recipient is also external.

This isn't easy to explain but I'll try. Picture a 3 company scenario. Person from company A sends a meeting request to someone at company B. The person at Company B ropes in another person at Company C by forwarding the meeting invite. That invite will be sent with the originator's address as the Sender. With more and more companies doing anti-spoof enforcement(DMARC, etc), this means the message is going to be blocked.