Yet all of the original messages are still in my Sent folder even though read.
OK, it's simple, I think.
You send a message/feedback to an author. The message is inserted into the database and it appears in your sent box and it appears in the author's inbox. Before the author opens the message, its default status is 'Unread' (displayed in bold).
Once the author reads your message, it's marked as 'Read'.
Now, if the author archives the message after he/she reads it, whether they replied or not, it stays in the database. It shows up in the archive box for the author and shows in the 'Sent' box for you.
However, if the author chooses to delete the message (regardless if they reply or not), then the message is removed from the database and it no longer shows for you in the sent box nor for the author.
If the author keeps the message after reading it (archive or inbox), then it's kept for one year. At the one year mark 'Read' messages are deleted from the database and they'll disappear from the author's inbox/archive and from your 'Sent' box.