I hate to necro a thread, but it's been five years and this is still the exact same question I have.
Our team is flooded with email notifications of all types and it's impossible to set up an Outlook filter because we have no way to modify the subject line in an Outgoing Email Template. (We've tried searching for specific text in the bodies of the email, but since every email has a copy of the history then eventually near the bottom you'll hit the filter text.)
Just something as easy as "New Ticket" or "Updated Ticket" in the subject line would do wonders for visibility.
If this has been added to a feature request, do you have a link?