Apple Mail

Hi,

Regret I cannot find a suitable topic that covers Apple Mail so have just selected Catalina.

I have just used import mailbox in Mail - excellent suggestion through Support Community - to pull in all my mbox folders containing emails created on a Windows PC. I am now showing around 2500 unread emails. There is a facility in Mail Mailbox which reads 'Mark all messages as Read'. Trouble is it is grayed out. It does not matter whether I start at the folder level or drill down to an individual email, it still remains grayed out. Is there a way I can mark all as read so that I can remove the 'new incoming emails' number from the Mail icon in the Dock ? Thanks