Simon Willison wonderful Datasette plus the mbox-to-sqlite extension is exactly what you want.
https://datasette.io/
https://github.com/simonw/mbox-to-sqlite
This is good but the UI is nothing like a regular mail client, so would be difficult to use for non-technical users.
This is good but the UI is nothing like a regular mail client, so would be difficult to use for non-technical users.