Add mail box as secondary to existing mai box
Have had a number of issues with 'Mail' which Apple Support have been approached for help, although do not think they have been resolved.
Anyway today whilst sorting incoming emails, in 'Favourites', 'All inboxes', to either move into already created boxes or delete. Decided I needed to add a secondary box to an existing box, but 'Create a New Mailbox' is actually not showing Mail Box in list shown to create.
Have done this numerous times before, believe using '+' at bottom of page to add... it's not there anymore!
What I am missing here?