Hi guys,
I believe you are describing what I am after - the possibility of addressing different Tickets that are already in the system. Here is the scenario:
- Someone created a ticket. The engineer took it and started to work on the case.
- Instead of using the Email button and starting new email communication, the engineer used their Email Client (Outlook) and initiated communication with 3rd party.
- If we can reply from the Email Client in a way that we hit the HubSport ticket so that then we can continue from there the communications - we have accomplished the goal.
Some refer to the functionality as Forwarding Address/Email. Microsoft has implemented similar functionality in its Teams allowing you to generate a Forwarding Email that can be used to forward messages to particular Teams Channels:
The tickets as entities already have their unique identifiers. So, HubSpot, you only have to do the “How to Address it” part 
Fingers crossed!
BR,
Dragomir Dragnev