Smart Mailbox has phantom unread message
Recently a few of my smart mailboxes are showing unread messages when there aren’t any. I have done all the fixes I could think of and find references to including;
Rebuilding the mailboxes.
Selecting all messages in a mailbox and marking them unread and then selecting all and marking read. (which worked until I closed the Mall app and then re-opened it)
Deleting the smart mailbox and then recreating it.
Thanks in advance for any help.
MacBook Pro with Retina display, macOS High Sierra (10.13.4)