As we all know Microsoft applications stores emails in PST format, so it's best to backup your Office 365 emails, attachments, and other items in .pst format. There are two ways using for backup:
1. You can export Office 365 mailbox to PST using ediscovery wizard.
2. Use the PowerShell command to backup the mailbox.
No matter the solution you pick, the result will be 100% valid, it's just a matter of technical expertise once have to perform such a solution.