This is something that we have now built a custom solution that we have deployed to a number of portals for where we can disassociate and associate emails based on an email subject or email contents, e.g. include the ID of the Deal/Ticket in the subject line or contents.
We are looking to add a Micro App to the Marketplace in the next few months to allow this to be configured direct, but in the meantime, if you have a desperate requirement you can reach out to hypeanddexter.nz - but HubSpot it would be great to resolve this please