Exchange Online Archiving for Exchange Server is your plan B when emails go missing in a users mailbox. What is great is this service can be applied to Microsoft Office 365 licenses or assigned to on-premise mailboxes.
Unlimited Archive storage quota
"The unlimited archiving feature in Office 365 (called auto-expanding archiving) provides up to 1 TB of storage in archive mailboxes in Exchange Online. Each Exchange Online Archiving subscriber initially receives 100 GB of storage in the archive mailbox. When auto-expanding archiving is turned on, additional storage space is automatically added when the 100 GB storage capacity is reached."