If you are looking for a professional method to convert Maildir & Maildir ++ files into another formats(PST, EML, PDF, HTML, CSV, Yahoo, Gmail, MBOX, vCard, Thunderbird, and many more.) then try CubexSoft Maildir Converter. It gives options  to convert unlimited Maildir email files in single round. Also, the GUI Interface of this Maildir Tool very simple to use. The app works effectively on Windows Operating Systems like Windows 10, 11, 8, 7, 8.1, etc. The application provides trial facility which enables file migration of 25 Maildir files without any charges.