Thought this article by Terry Zink is interresting enough to share here: https://blogs.msdn.microsoft.com/tzink/2017/06/22/an-update-on-the-forwarding-email-problem-in-offic...
The issue with signing messages forwarded via Transport rules and DL expansion should now be resolved in ExO. The same should apply to any messages that are redirected before hitting the mailbox and the MAPI layer. Microsoft is still working to address the "mailbox forwarding" scenario.
