Unread Sent emails????

My email client is Windows Live Mail (WLM) 2012. Yes, I know; it's old and no longer supported, but it's the best email client I've tried. However, every so often, an email that I sent. . . and is in the Sent folder. . . shows as "unread". Huh? What? I SENT the email. I don't need to read SENT emails. Why is it showing up as unread????

My email protocol is POP3. I tried IMAP. That made no difference in the occasional unread sent email, and I went back to POP3 because you cannot delete old emails with an IMAP account on Comcast; anything you delete re-downloads. And Comcast provides no automatic delete option (e.g., delete emails older than 90 days) and no easy way to manually delete emails after a specified date.