If you have an MBOX file but don’t have a relevant program to open it then take the help of MailsDaddy Free MBOX Viewer. This freeware application allows users to open single or multiple MBOX files at once. Users can also use the on different email client MBOX files and open or read the data without any restriction. This GUI tool supports all MBOX, MBS and MBX based email clients and runs on all Windows OS. You can also upgrade the tool if you want to migrate data into PST, Office 365, EML etc.
Please read the originial post here: https://www.mailsdaddy.com/free-mbox-viewer/