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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: Finance
Technology Used: Salient Preserve

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).

Of course, the cost of 9 500 Microsoft licenses is considerable. The bank challenged Cloud Essentials to find a more cost-effective solution that would:

  • meet their stringent security and compliance requirements
  • support future eDiscovery and investigation
  • be low maintenance (leavers’ mailbox data rarely, if ever, needed to be accessed)

Our Solution

After assessing and discarding several other popular solutions like Hubstor (for whom the bank did not meet the minimum 50TB volume requirement), Cloud Essentials 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.

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

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.

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


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

By adding future leavers’ mailboxes to the same repository in annual migration batches, the bank will also be able to free up and reuse Microsoft licences moving forward. (Salient will be on hand to enact those migrations and perform any other bulk service on their data that they may require.)

In the meantime, 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.