Right click the folder, select properties and then the repair button

Matt said

Right click the folder, select properties and then the repair button

This did not solve the problem. Still shows 63 messages.

is there a sub-folder that is collapsed. A little black triangle beside the folder name?

Check that View menu (Alt+V) > Threads is set to all

Matt said

is there a sub-folder that is collapsed. A little black triangle beside the folder name? Check that View menu (Alt+V) > Threads is set to all

Threads were already set to all and there are no black triangles.

I'd suggest you also check View|Folders is set to "All".

And that your quickfilter settings are all off or clear.

Where is the button to mark all as read? Did this arrive by way of an add-on? I don't think it's a standard feature of Thunderbird.

Zenos said

I'd suggest you also check View|Folders is set to "All". And that your quickfilter settings are all off or clear. Where is the button to mark all as read? Did this arrive by way of an add-on? I don't think it's a standard feature of Thunderbird.

Found the messages causing the problem when I set the View/Folders to "all"! Thanks! It was not a button, but a right click to mark all as read.

There used to be a pair of buttons, with arrows pointing left and right, for scrolling round the various folder views. They were removed at some point because it seemed to the developers that they confused too many users. I'm not at all sure that having them available through a menu that is itself hidden is any improvement.

The arrow buttons can be reinstated by means of the Folder Pane View Buttons add-on. Personally I find it useful to be able to switch quickly between All, Unread and occasionally Unified or Favourites.

Mark All As Read can also be accomplished by shift+c. You can of course create a button for yourself using a suitable add-on, so I wondered if that was what you were using.