Using Mac Mail, is it possible to export mail based on different criteria ? Ideal criteria would be 'messages older than ?' and export / move them to a backup mailbox. Export an existing mailbox isn't what we're looking for.
Not via the "Export Mailbox…" menu item but it's a simple enough thing to create a "Smart Mailbox" with the criteria requirement, move the messages to a new, normal mailbox and then export that mailbox. I don't think exporting a Smart Mailbox will work properly.