Back again*.
Suddenly Mail restarted connecting to my Yahoo mailbox and downloaded all my unread mail...but... now, when I try to get my mail it connectes to my online mailboxes, the mail in the mailboxes gets marked as read but does not download to my Mac, so... because the mail has been marked as 'read' I then have to log-on to my online mailboxes to read it. Sheesh!

How can I re-install Mail to try to sort this rubbish out?

*earlier message: