Are you looking for a method of Mail Backup from Office 365 to MBOX you should try the
eSoftTools Office 365 Backup Software. They have multiple salient and advanced features like the selection of multiple folders at a time, Accessible in All kinds of Windows OS, You should select the Date by any specific Date, Their Software gives you a chance to export Office365 Data into more than 9+ Different formats within 2-3 Steps. For all the users they offer Free Updates, 30 Day Money Back Guarantee, and a Free Demo Trial in which you get the 20 items free exportation into the most understandable format for you.
Read more:-
https://www.esofttools.com/blog/how-to-take-mail-backup-from-office-365-to-mbox/