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.  

Exchange Online Arching Plans

Microsoft Exchange Online Archiving is a Microsoft Office 365 cloud-based, enterprise-class archiving solution that comes included with Exchange Online, Office 365 business plans, Microsoft Exchange Server 2019 - 2013 and Enterprise Online  plans. 

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."

We have provided a few direct links to help you with your organization product planning, Each individual service feature can be review under the Exchange Online Archive Service Description. Once you have decided on the best plans for your organization it's time to Enabling New Office 365 Services.