How to Fix Unknown Error when Importing PST into Office 365?

Importing PST files into Office 365 can be a complex and time-consuming process. It also requires some level of technical expertise to use PowerShell and various features of Microsoft 365. There are some challenges involved in the manual process, whether it is network upload or drive shipping. Sometimes, unknown error occurs during the PST import process. Such an error appears in the LOG file when the PST import process fails with 0 items imported and 0 items skipped. This can disrupt the migration and may leave you looking for solutions.

In this guide, you will learn to troubleshoot and resolve the unknown error while importing PST files into Office 365.

Possible Causes of Unknown Error during PST to Office 365 Import

There could be a few reasons that can trigger the unknown error. However, the error does not appear during the import process. Rather, you notice that no PST file is imported into Office 365. When you check the log, it says unknown error at the bottom or in between the log file text.

Since there is no specific error code, it makes it difficult to pinpoint the exact cause. Thus, you will have to rely more on the guesswork and look at all the critical points or steps that are involved in the manual PST file migration from local storage to Office 365 to resolve the error.

Troubleshooting Steps to Resolve the Unknown Error

Follow the below steps to troubleshoot and resolve the unknown error that occurs during PST to Office 365 import process via the Network Upload method.

Step 1: Check the PST File for Corruption

Corrupted PST files can trigger unknown errors during the import process. To avoid this or other errors during the PST import process, you must check the integrity of the PST file. For this, you can use the ScanPST.exe tool provided by Microsoft. This utility is located in the Microsoft Office installation directory.

The tool scans and repairs any errors or corruption within the PST file and helps you ensure that the PST file is free from bad items, errors, or inconsistencies that may lead to unknown errors in Office 365.

The steps to scan and repair PST file are as follows:

• Run the ScanPST.exe tool and click Browse to select the PST file.
• Click Start. The tool will scan the PST file for errors.

• After the scan, click Repair.
• The Inbox Repair tool (SCANPST.exe) will start repairing the errors and inconsistencies found in the PST file.
• After the repair process, the PST file is ready for import into Office 365.

You can scan all your PST files that you want to import into Office 365 using SCANPST.exe.

Step 2: Ensure the PST File is not Password-Protected

Password-protected or encrypted PST files cannot be imported into Office 365 without entering the correct password. If you encounter an unknown error during the PST import process, make sure that the PST files are not password-protected. To check this, you can try to open the PST file in Outlook. If Outlook prompts you to enter the password, the file is protected.

You can open the PST file in Outlook after entering the password and then export the mail items from the open PST file to a new unencrypted or non-password-protected PST file using the Import/Export wizard.

Step 3: Make Sure PST Files are Smaller (>10 GB)

Large PST files can also trigger unknown errors during the PST import process. Before you upload the PST files to the Azure Blob Storage, make sure:

• The PST files are smaller, preferably >10 GB in size.
• Consider splitting the large PST file into smaller PST files using the Outlook Import Export wizard or a PST file splitter tool.
• If the PST file is in ANSI or another format, import or open it in Outlook and export the mail items to a new PST file. Then, try uploading and importing the PST file into Office 365.

Step 4: Check the CSV Mapping File

The information you share via the CSV mapping file is one of the most important pieces of information that Office 365 uses for importing the PST file from the Azure Blob Storage into the mapped user mailboxes. If there is a mismatch, typo, or incorrect entry in the CSV file, it may lead to an unknown error. Although Microsoft PST import service validates the CSV file, it only validates the columns and rows header tags. It cannot verify or detect any typos you made in the mapping file related to the PST file name, location, or destination mailbox names or folders. You can download this sample copy of the CSV and then carefully enter the required information in the CSV file.   

After the repair and following all the above steps, you can try again to import the PST files. If the unknown error persists, it’s recommended not to invest more resources or time into troubleshooting. Instead, use a PST to Office 365 migrator tool, such as Stellar Migrator for Outlook, to export all your PST files to Microsoft 365 (Office 365) with complete integrity.

With this tool, you don’t need to worry about the corruption, errors, or size of the PST file. You don’t need to create any CSV file. All you need to do is,

• Select and scan the PST file.
• Choose the Microsoft 365 configured account in which you want to import the data .
• Click ‘Migrate’ to export the PST to Office 365 with complete integrity.

It’s an easier, more convenient, and cost-effective way to import PST files from local storage into Office 365, without encountering the unknown errors.

Conclusion

The unknown error when importing PST into Office 365 is a critical error that can leave many scratching their heads as it does not appear during the import process. Rather, you need to check the LOG file post import job failure to learn the reasons behind the unknown error. To avoid the hassle and ensure consistent and complete PST migration from your local storage to Office 365, use Stellar Migrator for Outlook software. You can use the software to simultaneously upload multiple PST files from multiple systems and multiple locations in a few clicks.