Completing a Microsoft 365 migration does not necessarily mean all business email is under central control. Unmanaged PST archives may still be scattered across devices, home-working environments and the network, creating significant security, compliance and eDiscovery risks. This article explains how organisations can identify and migrate these legacy PST archives into Microsoft 365 with a secure, phased approach.
The risks of unmanaged PST files
As organisations adopt Microsoft 365, Microsoft Purview and AI-powered services such as Microsoft Copilot, centrally governed data is becoming ever more important. Hybrid working has also increased the number of devices and locations where business information may be stored.
Historical emails held in PST files are only covered by Microsoft 365 retention policies, eDiscovery workflows or other Purview capabilities when they are imported into Microsoft 365. They may also lack the associated access controls, monitoring, backup arrangements and information-protection measures.
For example, if a laptop is lost, stolen, damaged or affected by ransomware, locally stored PST data could be exposed or become inaccessible. Scattered archives can also make information requests, eDiscovery and regulatory investigations more time-consuming because teams may need to identify and search files across multiple endpoints and user accounts. There is additional risk for organisations handling sensitive or regulated information.
Why PST files remain
PST files were once a practical way to manage large mailboxes, support local archiving and work around the limitations of older email systems.
This means many organisations are now dealing with years of accumulated data rather than a single, clean migration project. The challenge is to bring that legacy content under control without disrupting active users or losing important records.
Why migrate PST data to Microsoft 365?
Importing PST into Microsoft 365 gives organisations a centralised location for legacy email and makes the data accessible through supported Exchange Online and Microsoft Purview capabilities.
Of course, those tools only work when the data is actually within Microsoft 365. Migrating PST files closes the gap between where the data is stored and where it needs to be for compliance and oversight. Incremental migration also means the process can be phased, reducing disruption for active users.
How the migration can be managed
Stellar Migrator for Outlook is designed to make this transition secure and manageable. It runs locally on the administrator’s machine, so data does not pass through a third-party cloud server during migration. It supports both PST and OST files, including orphaned OST files from decommissioned Exchange servers, and does not require Outlook to be installed.
The software supports files very large Outlook Data file (OST/PST) migration w with larger files automatically split. Mailboxes can be matched automatically to Microsoft 365 accounts, with manual override available where needed. Administrators can also apply date and folder filters so only relevant data is transferred, including email, contacts, calendars, tasks, notes and journals.
Incremental migration means jobs can be run more than once without duplicating data, which is useful for phased rollouts. Concurrency settings allow multiple mailboxes to be migrated at the same time, while priority controls let teams move business-critical users first. Auto Registration via Entra ID uses OAuth 2.0 and Microsoft Graph API, supporting secure modern authentication without legacy credential handling. A free demo is also available, allowing teams to preview recoverable data before committing to a licence.
A practical migration process
The process is straightforward for administrators and involves the following steps:
- Create a migration job, select the source PST or OST files and choose the Microsoft 365 destination.
- Connect the migration tool to Microsoft 365 using the approved authentication method.
- Review the target mailboxes, maps accounts and apply any required date, folder or data-type filters.
- Start the migration, monitor progress from the dashboard and export a report when the job is complete.
Closing the gap
PST files may be an established part of an organisation’s Outlook history, but they can create modern security, governance and discovery challenges when left unmanaged.
Importing PSTs to Microsoft 365 gives organisations a centralised, encrypted, and auditable environment for legacy email. There, it can be governed with retention policies, compliance tools, and eDiscovery capabilities designed to support modern risk management.
Find out more about Stellar Migrator for Outlook.
