I submitted a feature request on the Bugtrack. If you think it would be helpful to you go ahead and add your "me too". http://www.istechsupport.com/bugtrack/view.php?id=266
 
Here's what I wrote:
 
Because we deal with repeat customer is can sometimes be a real trial figuring out what an email reply from them is referring to. For instance if they reply to the email sent from DBA and say "Looks good - go ahead" we have no idea what they're talking about unless we look at the Date/Time originally sent and then find the BCC copy that matches up.
 
We manually enter the document # in the subject, but it's a pain to remember, a pain to do, and subject to data entry errors.  

You have to have an account and be logged in. Then there is a place at the bottom to add a note.
 
In this case, they have apparently decided it's worth doing and have already assigned it to someone.