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Email Archive and PST Migration to Microsoft 365

Email archive migration moves historical mail from legacy archive systems, journals and PST files into Microsoft 365, usually into Exchange Online archive mailboxes, while preserving retention, legal hold and chain of custody. For PST files, Microsoft Purview offers a free network upload import and, under Enterprise Agreements, a paid drive shipping service; legacy archive platforms usually need a specialist migration tool that reads the archive directly.

Archives are often the largest and most compliance sensitive part of an email migration. RockTeam handles them as part of our email migration services.

Types of Email Archive Migration

SourceTypical targetApproach
PST files on PCs and file sharesPrimary or archive mailboxesDiscover, collect and import with Microsoft Purview PST import
Legacy archive platformsExchange Online archive mailboxesArchive migration tooling that extracts items and rehydrates shortcuts
Journal archivesCompliance storage or inactive mailboxesPreserve chain of custody and retention for regulatory records
Archives of departed employeesInactive mailboxesKeep searchable for eDiscovery without paid user licenses
Lotus Notes archivesExchange Online archivesNotes to Office 365 migration tooling

PST Import to Microsoft 365: Network Upload vs Drive Shipping

FactorNetwork uploadDrive shipping
CostFreePaid service
LicensingAll commercial agreementsEnterprise Agreement only
VolumeUnlimited, network dependentUp to 10 TB per drive, 10 drives per job
SpeedSeveral hours per TB to uploadAbout 7 to 10 business days after the drive arrives
Best forMost projects and ongoing importsVery large one-time imports

PST import limits to plan around

  • PST files should be no larger than 20 GB each.
  • Items over 150 MB are skipped.
  • Imports run at roughly 24 GB per day per mailbox, not guaranteed.
  • A single import into an archive mailbox is limited to 100 GB; larger archives are split across imports.
  • Imported mailboxes are placed on retention hold until you change it, so retention policies do not purge old items.

Email Archive Migration Steps

  1. Define what must be kept. Confirm retention periods, legal holds and records that must move versus data that can be retired.
  2. Discover archives and PST files. Find PSTs on PCs, laptops and file shares and inventory legacy archive stores and journals.
  3. Clean up. Remove duplicates and out of policy data where your retention rules allow.
  4. Prepare targets. Enable archive mailboxes, set retention policies and plan inactive mailboxes for leavers.
  5. Migrate. Import PSTs and extract legacy archives in batches, validating item counts and audit logs.
  6. Remove old dependencies. Remove archive shortcuts and stubs, block new PST creation if policy requires, and decommission the old archive after sign-off.

Compliance and Chain of Custody

For regulated organizations, the migration must prove that every item arrived intact. Keep item counts, error reports and audit trails for each batch, migrate journal archives so they remain searchable, and confirm legal holds are in place in Microsoft 365 before the old system is retired.

Common Archive Migration Mistakes

  • Leaving PST files behind on laptops and file shares.
  • Ignoring shortcuts and stubs that point to a legacy archive users still click.
  • Letting retention policies purge imported mail by lifting holds too early.
  • Underestimating volume and throughput, especially for multi-terabyte archives.
  • Skipping verification reports needed for audits.

Need to migrate email archives or PST files?

RockTeam migrates PST files, journals and legacy archives into Microsoft 365 with retention, legal hold and verification reporting in place. RockTeam is an official HCL Business Partner and a long time Microsoft collaboration specialist. Start with a no cost assessment.

See Email Migration Services

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Email Archive and PST Migration to Microsoft 365 FAQs

Moving historical email from legacy archive systems, journals and PST files into a new platform such as Microsoft 365, while preserving retention, legal hold and chain of custody.

With the Microsoft Purview Import service, either by uploading PST files over the network for free or, under an Enterprise Agreement, by shipping drives to Microsoft.

Yes. The import mapping can target a user’s archive mailbox instead of the primary mailbox, up to 100 GB per import into an archive.

Microsoft places imported mailboxes on retention hold so policies do not purge old items. Review retention settings before lifting the hold.

Legacy archives usually need a specialist migration tool that extracts items, preserves metadata and removes shortcuts, with verification reports for compliance.