BLR MBOX Converter to Import MBOX to Yahoo Mail

"The best option for importing MBOX emails into a Yahoo Mail account is the BLR MBOX converter tool. The program will save all of your emails from your MBOX file into your Yahoo Mail account with only a few clicks.

How to import emails from MBOX files into a Yahoo Mail account

Launch the MBOX Converter tool.
Include the files in MBOX.
From the several options for saving, choose Yahoo Mail.
After entering your Yahoo Mail login information, click the Convert button."