We can find different solutions here but we need something that ensures to backup emails, contacts, calendars & tasks from Office 365 mailbox to PST, OST, MBOX, etc. Then, use the MacSonik Office 365 Backup Tool. It has all the requirements a software needs when migrating Office 365 to multiple email clients. This tool offers a Mail filter & task filter option to download Office 365 Mailbox to PST from a specific date range.