N.B. when you come to looking at your own sent items folder, the terms Read and Unread are confusing compared to the previous version of the forum.
They relate to your actions, not those of the recipient(s)

Re the top message, I have not read (opened) the message I sent, the second one down, I have opened it and digested the contents. i.e. that column is an Action for the mailbox owner, not what the recipient has done with the message
[ in reality, I did open it, considered the contents, and then re-marked it as unread to my own eyes]
Really confusing to me, so I wonder what those who don’t have English as a first language make of it.
On the old forum, and many others, that terminology referred to whether the recipient had opened and read the message you sent.
Bill