Retain the Original Subject
Depending on where you are, you may be able to change the subject by typing a new one in the Subject line when writing your reply. But this may not be the best path to take. Rather than change the subject, make it clear that you’re continuing an old thread and not starting a new one by including the previous subject line with the new one. If the original subject was “New cloud form discovered,” and you want to change it to “The finest English umbrella,” the complete new Subject line could be “The finest English umbrella (was: New cloud form discovered).” You can, and probably should shorten the original.
Caveats When Changing a Subject
If you choose to change the subject:
Don’t edit any of the earlier content or thread messages.Don’t remove the recipients of the earlier emails.State why you are changing the subject to avoid confusion.
Sometimes Starting Over Is a Better Choice
Changing the subject line to start a new conversation can lead to display problems for others and yourself. Email programs and services may lump together the wrong messages in threads. To avoid this problem and the likelihood of being seen as threadjacking, which occurs when someone takes over a thread or email discussion and intentionally posts on a subject unrelated to the original post. They create a new message with a new topic instead of starting with a reply.
