Apple Mail (Catalina)

It will be deleted. Smart mailbox is for organizing mail not for saving mail.

For more about Smart mailboxes: https://support.apple.com/guide/mail/use-smart-mailboxes-mlhlp1190/mac

https://support.apple.com/guide/mail/move-or-copy-emails-mlhlp1000/mac

That is interesting.

I don't myself have any Smart mailboxes. Instead, in the left column, I have extra folders under a section shown as 'On My Mac' some of which have sub folders called, for instance, 'Receipts' or 'Registrations'.

I manually drag relevant emails into those. I'm hoping that these don't get removed if my 'Inbox' gets cleared? Which is likely because it's getting pretty stuffed.