Customisable Subjects in Emails

Email subject lines need to be customisable. Customers receive many different email threads due to diverse subject lines and will have multiple conversation threads happening at once. This is a confusing experience for them that needs to be optional. We originally didn't notice this as Collins email consolidates them all. But now that we use a CRM instead, we have discovered that there are actually multiple threads happening at once. Apart from being a poor customer experience, it creates a very difficult problem to resolve in the CRM for consolidating threads to a single support case.