Found this, and it help. Would have prefered a more out of the box solution, that a reg hack: crmtipoftheday.com/.../
"Hi All,
I liked reading your post. I think I might have found a way to smart hack the tracking for Replies and Forwards when using the Microsoft Outlook desktop client on Windows to get it working like the old Com-Addin. I have tested this and so far it seems to work a treat. This allows the new Microsoft Dynamics 365 App on Outlook for Windows to auto track for all replies and forwards when the original email you are replying and forwarding has been tracked.
Add the following registry key:
[HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\Preferences]
“SendPersonalCategories”=dword:00000001
What this effectively does is add the “Tracked in CRM” category to your replies and forwards if the email you where replying or forwarding was in itself already tracked and had the category “Tracked in CRM”
Do some testing and you will see the same results on your end. The great thing about the old Com-Add in was the auto tracking of the forward and reply if the user tracked the inbound email to begin with. Was great as the user didn’t have to manually track the reply or forward. This hack solves the problem on Microsoft Outlook on PC.
Hope this helps others out there who are trying to move to Dynamics 365 App and finding some functionality has been lost on Outlook on PC."