Both Apple Mail and Thunderbird support the option of opening a previous email and then entering an “edit” mode where you can change the text, etc, and send it out as a new email. This is useful in instances where you send out a recurring email and want to keep the same list of “TO”, “CC”, and “BCC” recipients and just want to change something minor in the text.
I tried opening the message in a new window, I even tried moving the message to the drafts folder but there was no way to make emClient let me edit the message and resend it.