Migrate PST to Office 365 Without the Size Limit in 2026

Why Office 365 limits PST files to 20 GB

The built-in import path, Network Upload via the Microsoft Purview compliance portal, places every PST file in a private Azure Storage Container where it is staged for import into user mailboxes by Microsoft’s backend service. Microsoft recommends that files be 20 GB or smaller due to significantly slower performance and more challenging failure scenarios. There is also a separate limitation related to individual email items. An email larger than 35 MB may be skipped unless the mailbox’s MaxReceiveSize property has been manually increased to 150 MB beforehand. These limitations are designed around how the native import process handles PST data. Organizations that need to migrate PST files to Office 365 across dozens or hundreds of accounts are exactly the ones most likely to be sitting on oversized archives, so the limit tends to matter most for the migrations where it’s least convenient.

Native Method to Import PST to Office 365 via Network Upload

If you have some PST files, then Microsoft’s native process is useful. It requires:

• Assigning the Mailbox Import Export role to the account running the migration.
• Downloading AzCopy and using it to upload PST files to a temporary Azure Storage location.
• Building a CSV mapping file that pairs each PST file with a target mailbox.
• Creating and submitting a PST Import job from the compliance portal.

Once the job starts, Microsoft imports the mapped files into the correct mailboxes automatically.

Where the Native Method Breaks Down

• File size: anything over 20 GB needs to be split before upload, which means extra software and extra time.
• Daily throughput: Microsoft’s own guidance points to roughly 20 to 24 GB of import volume per day, so large batches stretch across multiple days.
• Mapping limits: The CSV file supports a maximum of 500 rows, so large user counts require multiple mapping files and multiple jobs.
• Sequential processing: several PST files targeting the same mailbox import one after another, not in parallel, adding time to already long jobs.
• Outlook Not Needed: The PC running the migration does not need an Outlook installation.

For an admin with three or four PST files, this is manageable. For an admin looking to migrate bulk PST Files to Microsoft 365 across a whole department, it expands to a multi-day effort involving manual tracking and management.

How DRS Softech Migrates Large PST Files to Office 365

Users searching for ways to export large PST files to Office 365 can use a dedicated migration workflow, such as the DRS Softech PST to Office 365 Migration Tool, can help to simplify the migration process. Microsoft’s built-in method for importing PST data has a 20 GB limit per file; the tool includes an explicit workflow for migrating PST data to Office 365, ideal for large and oversized PST files, suitable for migrations where PST archives are approaching or surpassing the native 20 GB-per-file limit, making it suitable for migrations involving PST archives that reach or exceed the native 20 GB-per-file threshold.

The workflow itself is short:

1. Select a single PST file or an entire folder for bulk migration.
2. Preview the folder structure and mail items before starting the migration.
3. Sign in with Office 365 credentials and choose the destination, such as a mailbox, public folder, or online archive.
4. Apply a date filter or attachment-related filter if only specific data needs to be migrated.
5. Start the migration.

Key features

A few features matter more once file sizes and mailbox counts grow:

• Bulk Migration: Bulk PST file migration in a single workflow, instead of managing separate migration processes for each user.
• Remove Duplication: Duplicate removal based on available item details, helping prevent repeated imports from cluttering the target mailbox.
• Folder Hierarchy: Folder structure and metadata preservation, helping maintain the original organization when you migrate PST data to Office 365.
• Impersonation Support: It’s for importing into multiple primary and sub-accounts without requiring separate logins for each account, where the required permissions are configured.
• No Outlook Required: No Outlook installation is required on the machine running the migration.

Best Practices Before You Migrate

• First, run the small test batch, even in a complete migration has hundreds of PST files.
• Verify before starting the import that every mailbox has an active Office 365 licence.
• Schedule large jobs outside business hours if bandwidth is shared with other applications.
• Keep and maintain a record of which PST files have already been processed; specifically, migration completion is multi-session.
• If your destination is Google Workspace, not Microsoft 365, the process is different enough to justify reading through this guide on how to migrate PST to Google Workspace before proceeding.

Conclusion

This is where the DRS Softech PST to Office 365 Migration Tool can help to simplify the migration process. Microsoft’s built-in method for importing PST data has a 20 GB limit per file; the tool includes an explicit workflow to migrate PST to Office 365, ideal for large and oversized PST files, suitable for migrations where PST archives are approaching or surpassing the native 20 GB-per-file limit.

Frequently Asked Questions