Mail recreates inbox, sent, draft, junk and trash boxes “On my mac” tab.
After updating to Big Sur Mail.app automatically creates a replica of all my 4 account boxes under
the “On my mac” tab. It clutters the sidebar with at least 20 duplicate boxes I don’t want. I tried deleting them but I lose all the mails in the account. Plus as soon as a new email appears on an account the box is recreated "On my mac".
iMac 21.5″, macOS 10.13