I concur with David. Rather than using a temporary directory as a Maildir folder:

    with tempfile.TemporaryDirectory() as tmpdir:
        with mailbox.Maildir(tmpdir, create=True) as box:
            ...

create a Maildir folder in a temporary directory:

    with tempfile.TemporaryDirectory() as tmpdir:
        with mailbox.Maildir(os.path.join(tmpdir, 'mail'), create=True) as box:
            ...