I just downloaded and tested this add-on and it works PERFECTLY, just like the old 'Edit Email Subject' that I used so much! THANK YOU!!! I will proceed to make a small contribution now to show my appreciation for your work.
Folks, this add-on is very useful because correspondents frequently neglect to put something in the subject line at all, or write something useless (like 'meeting'), so I edit the line to write the meeting date, time, place, so I can see at a glance what's what w/o having to open the message. Or when I order something on line, I can add 'RECEIVED' to the subject line so I can see instantly if something's still pending or not.
Such a useful add-on!! Thank you.