You can easily upload your MBOX file to Yahoo Mail using the
eSoftTools MBOX Converter Tool. This tool helps you convert and upload MBOX files to Yahoo Mail, Gmail, Office 365, and other cloud services. It's simple to use, even if you're not very tech-savvy. You can try it for free by importing up to 25 emails from your MBOX file to Yahoo Mail before deciding to buy the full version for unlimited conversions.
Read More:- Import MBOX File into Yahoo Mail