To save Yahoo Mail to Office 365 account, you can try CubexSoft Yahoo Mail Backup Tool that is helpful solution to directly import Yahoo Mail to Office 365 account. It can simply save Yahoo Mail data to O365 with attachments. The utility supports to save Yahoo Mail to O365 with metadata, formatting, images, etc. You can try it that can export 25 emails from Yahoo Mail to Office 365 account at free of cost.