mailbox "on my mac" location

Hi guys,

A very long time ago I created a seperate folder/mailbox called "Later" in Mail.

The location is "on my mac".

I cannot figure out where I can edit this folder.

Google searches aren't satisfying.

Every time I ask to "delete" (Mail - mailbox - delete mailbox...) the folder disappears for a while, but returns when there's a new mail that answers the settings I once made.

So I cannot delete it for good, it always pops up again.

Any ideas why and how I can fix it (macOS Big Sur)?

Thanks!

Britt