How to Import an OST File into Microsoft 365
Importing an OST file into Office 365 isn't required simply because you're moving to Microsoft 365 —it's typically needed when mailbox data is stranded, orphaned, or otherwise unavailable in the live account, such as an old Outlook profile issue or offline data you need to recover. OST files are associated with their original Outlook profile and are not directly supported as an import format by Microsoft 365. When an OST becomes orphaned, inaccessible, or contains data unavailable in the live mailbox, a recovery or migration tool can be used to extract and migrate the recoverable mailbox data.
EdbMails OST to Office 365 Migrator enables direct migration of recoverable OST mailbox data, including emails, attachments, contacts, calendars, tasks, and other supported items, to Microsoft 365 without first converting the OST file to PST.
Microsoft's native Import service accepts PST files, not arbitrary OST files.
For an orphaned/offline OST, either convert to PST and use Microsoft's PST import route or migrate directly with EdbMails
The workflow is straightforward: choose the OST file, connect to the Microsoft 365 tenant, and select the required items or folders. You can migrate the recoverable mailbox data detected in the OST or limit the scope to selected folders and items.
Why Import OST to Office 365?
Importing an OST file into Microsoft 365 is mainly required when mailbox data is available in the OST but is no longer accessible from the original live mailbox. Common scenarios include:
- Moving to Office 365: Normal Exchange-to-Microsoft 365 projects should use mailbox migration from Exchange as the authoritative source.
Use OST only for stranded/orphaned users or missing local data.
- Changing to a New Computer: A new computer is not a reason by itself to upload an OST to Microsoft 365; sign in to the existing cloud mailbox and let Outlook create a new cache.
Recover the old OST only if it contains data unavailable in the live mailbox.
- Recovering Data After a System Crash: A system crash can damage the local OST file or leave it in an unreadable state.
If the OST was lost, restore/recover the file first. If it survives, then migration is possible.
- Native PST Import vs Direct OST Migration: Microsoft's built-in Import service works well for organizations standardizing on PST files, but it requires converting the OST to PST first and following its network-upload or drive-shipping process. EdbMails offers a direct route when you want to migrate OST data to Office 365 without that PST-staging step.
Direct vs PST-staged comparison
| Path | Workflow | Best for |
|---|---|---|
| EdbMails direct | OST - > Microsoft 365 | Stranded/orphaned OST recovery |
| Microsoft native PST import | OST - > PST - > Microsoft 365 | Organizations that require a PST-staged import workflow |
Choose Direct Migration or a PST-Based Route
A PST can be useful when a portable, reviewed copy is required. When Microsoft 365 is the approved final destination, direct migration can reduce the number of staging steps. Choose the route based on the required outcome, source condition and validation process rather than only because the source is an OST file.
For a complete decision path, see the OST to Microsoft 365 migration guide.
Migration prerequisites
Before importing an OST file to Microsoft 365, ensure the following:
- Offline OST file: Keep the source OST file available on the computer running EdbMails.
- Microsoft 365 account permissions: Use a Global Administrator account for Auto Registration, or a user account with Full Access rights to the target mailbox for Manual Registration.
- Target mailbox: Ensure the required target Microsoft 365 mailbox exists and is accessible.
- License and mailbox quota: Verify that the target user has the required Microsoft 365 license and sufficient mailbox storage quota for the data being migrated.
Steps to import OST to Office 365 with EdbMails
Step 1: Download and install EdbMails software
- Download and install the EdbMails application on your computer.
- Launch the EdbMails application on your system.
- Enter your registered email address and password, then click ‘Login’.If you’re a new user, select ‘Start Your Free Trial’ to proceed with the trial version.Note:The EdbMails free trial allows migration of up to 30 items per folder. A valid license is required to migrate beyond the trial allowance.
Click here to purchase the license from the EdbMails website.
After purchasing the software, just exit and restart the EdbMails application. Use the same email address and password that you used while purchasing to successfully activate your license.
- Select ‘OST Recovery and Migration’.
- Select ‘OST to Office 365’.
- You can retain the default job name or click the ‘New Job’ button to change the job name.
- Then, click ‘Next’ to continue.
Step 2: Browse and select the OST file that you want to upload
- Click ‘Browse for OST file'.
- Select the offline OST file from the local drive and then click on ‘Next’ to proceed. You can also share the OST file on a network with read/write permissions.
- EdbMails scans the offline OST file and identifies readable mailbox data. For a damaged or corrupted file, the recoverable result depends on the condition of the source, so allow the scan to finish before reviewing the available emails and other mailbox items.
- After scanning, the recoverable folder structure appears in the left pane. You can open folders and view detected emails, contacts, calendars, tasks and notes in the application even if Outlook is not installed on your computer.
Step 3: Preview and choose the mailbox
- Check emails, calendars, mailboxes, or folders, and click ‘Next’.
Step 4: Connect to the target Office 365 server
- Click the ‘Add New Connection’ button to configure a new connection to your target Office 365 server. If you’ve previously connected, simply choose that connection from the list and click ‘Connect to Existing’ to proceed.
- Choose the connection options you require and hit the ‘Next’ button.
- Select an authentication method and click on the ‘Login’ button to continue.
- Authenticate on Microsoft sign-in page.
- Choose one of the available methods to load the mailboxes. Alternatively, you can import the mailbox list using a CSV file for bulk selection.
Step 5: Map source and target mailboxes
- Choose the required mailbox mapping option.
- Where supported by the selected workflow and tenant permissions, EdbMails can assist with target mailbox creation and licensing. Verify the created mailbox and assigned license before migration.
- By default, EdbMails automatically maps mailboxes & folders between the source and target server. If required, you can manually map source mailboxes to specific target mailboxes for more control over the migration
Step 6: Initiate the import of all your mailbox data from OST to Office 365
- Click on the ‘Start Migration’ button to start the migration.
- Migration is underway, and you can track its progress with the progress bar. You can pause or resume the process anytime you want. After the migration process is completed, EdbMails will show a confirmation message for the successful completion of the process.
- Download and install the EdbMails application on your computer.
Validate an OST Import into Microsoft 365
| Stage | What to confirm |
|---|---|
| Before migration | Target identity, mailbox status, quota, access authorization and selected source folders. |
| During migration | Job progress, throttling messages, skipped items and any retry requirements. |
| After migration | Correct mailbox and folder destination, sampled mail, attachments, contacts and calendar items. |
| Handover | Migration log, selection rules, exceptions and source-retention decision. |
Why Use EdbMails to Import OST File to Office 365?
- Import OST File Directly: EdbMails lets you import an accessible offline or orphaned OST directly to Microsoft 365 without Outlook, Exchange connectivity, or the original user profile. This avoids the separate OST-to-PST staging step and reduces the number of migration steps.
- Recovery of Readable OST Data: EdbMails scans offline, orphaned, and inaccessible OST files for readable emails, attachments, contacts, calendars, tasks, and notes. It migrates recoverable mailbox data while preserving supported folder hierarchy and properties, though severely damaged files may leave some items unavailable.
- Selective OST Data Migration: EdbMails lets you select the required mailboxes and folders before migration, so only the necessary OST data is transferred to Microsoft 365. Selective migration can reduce unnecessary data transfer and help control the amount of data migrated to the target mailbox.
- Easy, Simple Alternative to PST-Staged Import: Microsoft's native Import service is a solid option for organizations standardizing on PST files, though it involves converting OST to PST and following its upload process. EdbMails provides a guided direct-migration workflow for transferring recoverable OST mailbox data to Microsoft 365 without PST staging. Users can preview the recovered data, select the required folders, map the source to the target mailbox, and start the migration.
- Automatic Mailbox Mapping: EdbMails automatically maps source and target mailboxes and creates mailboxes on the destination tenant if needed during import OST File to Office 365. This can reduce manual mapping effort; review the source-to-target mapping before starting migration.
- Pause and Resume Migration: EdbMails allows pausing and resuming the migration when required. This provides additional control during planned maintenance or temporary network interruptions.
Frequently Asked Questions
Can Microsoft 365 import an OST file natively?
No. Microsoft's native Import service accepts PST files, not arbitrary OST files. An orphaned or offline OST needs to be converted to PST first for that route, or migrated directly with a tool such as EdbMails.
Do I need to convert OST to PST first?
Only if you're using Microsoft's native PST import route. EdbMails offers a direct OST-to-Microsoft 365 path that skips the PST-staging step.
Can I migrate OST directly without Outlook?
Yes. Direct OST-to-Microsoft 365 migration with EdbMails does not require Outlook. Outlook is only needed if you export the recovered OST data to PST instead.
Can I migrate multiple OST files to Microsoft 365?
Yes. On a single computer, EdbMails processes one OST file at a time. For multiple OST files, separate installations on multiple computers can process different files simultaneously and migrate the selected data to Microsoft 365.
What permissions are required to import an OST file into Microsoft 365?
EdbMails uses Modern Authentication with OAuth 2.0 to connect to Microsoft 365. The required permissions depend on the authentication method:
For Manual Registration, the EdbMails application must be registered in Microsoft Entra ID with the required application permissions. Refer to the EdbMails Manual Registration guide for configuration steps.
- Auto Registration: Sign in using an account with Global Administrator access.
- Manual Registration: You can use either a Global Administrator account or a user account with Full Access rights to the target mailbox.
Can I migrate only selected folders/date ranges?
You can choose specific mailboxes or folders during the migration steps above. For date-range filtering, confirm the available settings for the current migration workflow before starting the job.
What if the OST is damaged?
EdbMails scans the OST to determine what can be read and recovered. If the file is severely damaged, some items may be unavailable.
Can recovered OST data be migrated to an Online Archive mailbox?
Yes. EdbMails supports migrating recovered OST data directly to the Microsoft 365 Online Archive mailbox. You can select the required OST mailbox data and migrate it to the target user's Online Archive mailbox.
