I would like to recommend you the best third party solution with the eSoftTools OST Recovery software. This tool import OST file into Yahoo Mail without making any change of original structure. The software will keep the hierarchy of email files and folders after getting email from the OST file. This tool is based on the GUI Operating system for making it easy to understand by all kinds of users. It supports ANSI and Unicode. User can download a free demo version and convert 25 items to each Yahoo Mail folder.