If you want to backup of the yahoo mailbox then I would like to suggest you can also try Mailsbackup Yahoo Backup Tool to backup Yahoo email Accounts folder to hard drive, PC or webmail. save Yahoo emails in different forms, such as PST, EML, and MBOX, with this powerful tool. It offers complete data security and keeps folder hierarchy intact during the backup procedure. With an easy-to-use interface, the task can be done by both technical and non-technical users. You can choose which files to export and set up automatic backups. Control your emails with this dependable method.

Check for more details at:  https://www.mailsbackup.com/yahoo/