kmail continues to show read messages as unread

This morning kmail properly showed 3 or 4 emails received today as unread but 
improperly showed 30 emails received last month as unread.

Has been happening for months. Quite a bother.

I have heard that it might help if I did the following:

just trash
~/.local/share/akonadi*
~/.local/share/kmail*
~/.config/akonadi*
~/.config/kmail*
~/.cache/akonadi*

But I am afraid this might do permanent damage.  I need to keep the old emails.

Any suggestions?