Some Mac OS Mail Favorites are grayed and cannot be deleted

All of a sudden, some Favorites from the Favorite Bar in MacOS Mail appear grayed out and cannot be removed, dragged or deleted. I actually was able to set in the Favorite bar the same mailboxes that are grayed out and they work OK. I restarted the Mac but the grayed Favorites remained. Even though these grayed favorites are not working, they are a nuisance and I would love to remove them from the Favorite bar. I would appreciate very much help from anybody. Thanks!