Seems to be a great Add-On coming near to something (simple) I'm looking after since (years) I use TB, namely having messages in the Send-Folder initially with the flag UnRead.
FYI: I didn't try the AddOn as from the description it doesn't have the feature I'm looking after
Now, when I want to follow-up a sent message, I have to go to the Send-Folder, search for the message (my Send-folder is sorted UnRead/Read + Date) and mark the message Unread.
A variant of this AddOn could greatly do the job : when sending ask for Unread/Read and done.
Would be great !