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Cost-effective legacy mailbox archiving for Microsoft 365

Salient Preserve enables public bank to compliantly preserve leavers’ mailbox data without requiring additional Microsoft 365 licences.

Client Sector: Corporate
Technology Used: Salient Preserve
Cost-effective legacy mailbox archiving for Microsoft 365

The Challenge

A sizeable public bank approached Salient’s sister company, Cloud Essentials, with a common cloud migration challenge. The bank was transitioning to Microsoft 365, but wasn’t sure how best to handle the retention of their leavers’ mailbox data.

With 1000+ users and significant staff turnover over time, the bank had amassed data from over 9 500 inactive email addresses (more than 9TB of data, uncompressed) in their Mimecast email archives. The recommended method of preserving this data in Microsoft 365 was to assign an active M365 account to each leaver’s email address and store the data there (in-place archiving), but with 9500 to deal with, the licence costs were considerable.


  • Must meet the bank's stringent security and compliance requirements
  • Solution must support future eDiscovery and investigation
  • Solution should be low maintenance (leavers’ mailbox data rarely, if ever, needed to be accessed)


  • Salient solution met all required security, compliance and immutability requirements
  • We ensured the client was set up to deliver against future eDiscovery requirements
  • The client achieved cheaper per annum cost than Microsoft licences for every legacy user
The Salient Solution

After assessing and discarding several other popular solutions like Hubstor (for whom the bank did not meet the minimum 50TB volume requirement), our team introduced Salient Preserve.

Salient Preserve would enable the bank to store all their legacy email archive data in their own Microsoft Azure tenant’s BLOB storage. This would keep it entirely under their own control, subject to their own governance policies, with zero lock-in, and would be extremely cost-effective given the data’s low-touch nature.

Security, compliance and immutability would be ensured through a combination of Azure’s security capabilities and the BLOB’s account settings. Deleted items retention folders would also be set up to act as a failsafe for preservation purposes.

As for future eDiscovery and investigation, Salient Preserve’s intelligent ingestion capabilities would store the data, uncompressed, with a logical folder structure to streamline future imports and exports. Metadata would be enriched on upload with searchable attributes, including sender, recipients, message subject, date, size and attachments. A searchable subset (the first 2000 characters) of the message body would also be created.

This would enable the bank to quickly and accurately discover data using the Salient Preserve utility, producing results that could be seamlessly uploaded into Salient’s eDiscovery platforms (or other third-party solutions) for further investigation.

Long term storage concerns allayed

A single BLOB can accommodate up to 100TB of data, and multiple BLOBs can be used together if necessary, so storage volume would not be a concern now or in future.

The combined cost of the Azure BLOB storage and Salient Preserve utility proved cheaper per annum than restoring the legacy mailboxes to Microsoft 365 would have been.

Additional benefits that support compliance

Metadata would also be enriched with the bank’s retention policy data to support defensible deletion down the line, if necessary.

The bank has a secure, compliant, cost-effective and searchable storage solution for their legacy email archives, with a full audit trail and an easy and intuitive interface for their IT and Governance Teams to use.