Fix Outlook Import Errors After PST Migration
PST migration is a common way to move Outlook emails from one computer, profile, or account to another. However, the process does not always go smoothly. Users may face errors such as incomplete imports, missing emails, corrupted folders, duplicate messages, or Outlook refusing to open the PST file.
These problems can be frustrating, especially when the PST contains important business emails and attachments. Fortunately, most Outlook import errors can be identified and fixed with the right approach.
This guide explains the common causes of PST migration errors and the best ways to recover and import your emails successfully.
Why Do Outlook Import Errors Occur After PST Migration?
There can be several reasons behind a failed PST import. Understanding the cause makes troubleshooting easier.
1. Corrupted PST File
A PST file can become damaged because of improper shutdowns, storage problems, software crashes, or interrupted migration. If the file contains corrupted items, Outlook may skip them during import.
Microsoft recommends using the Inbox Repair Tool, commonly known as ScanPST.exe, to diagnose and repair PST errors.
2. Large PST File
Large PST files can also create performance and migration problems. Microsoft 365 recommends uploading PST files no larger than 20 GB because larger files can affect import performance.
If your PST is very large, consider splitting it into smaller files before migration.
3. Unsupported or Older PST Format
PST files can use different formats. ANSI and Unicode are the major types. Microsoft 365 supports both, but Unicode PST is generally preferred for modern Outlook and Microsoft 365 environments.
Older ANSI PST files can also create compatibility problems, particularly in certain Outlook for Mac scenarios.
How to Fix Outlook Import Errors
Step 1: Create a Backup of the Original PST
Before repairing or modifying anything, create a copy of the original PST file.
Never perform multiple repair attempts on your only copy. Keep the original file untouched. This gives you a safe recovery option if something goes wrong.
Step 2: Check Whether Outlook Can Open the PST
Try opening the PST manually in Outlook.
In classic Outlook, use the option to open an Outlook Data File and select the PST. Check whether folders, emails, attachments, and other items appear correctly.
If Outlook displays an error, the PST may need repair.
Step 3: Run ScanPST.exe
Microsoft's Inbox Repair Tool can diagnose and repair certain PST problems. Microsoft also recommends repairing corrupted PST files before uploading them again for Microsoft 365 import.
After the repair:
Open Outlook.
Check the repaired PST.
Verify important folders.
Confirm that emails and attachments are accessible.
Try the migration again.
Step 4: Check the PST Size
If your PST is very large, divide it into smaller files.
Microsoft states that PST files larger than 20 GB can affect import performance. Its documented import rate is approximately 24 GB per day, although actual speed can vary depending on server workload and other conditions.
Therefore, splitting a large PST can make migration easier to manage.
Check for Oversized Emails
Another important issue is message size.
Microsoft 365 can skip individual mailbox items larger than 150 MB during PST import.
Therefore, if some emails are missing after migration, check whether they contain extremely large attachments or embedded files.
Do not assume that every missing email means the entire PST is corrupted.
What If the PST Still Cannot Be Imported?
Sometimes ScanPST cannot fully recover a damaged file. In such cases, you may need specialized recovery or conversion software.
For example, eSoftTools Software provides email conversion and recovery utilities designed to work with formats such as EML and Outlook PST. Its EML conversion tools can convert multiple EML files into Outlook-compatible PST format and maintain email folders and properties during conversion.
If your migration started with EML files, converting the source messages into a properly structured PST can be another practical recovery approach.
Can a Free EML Converter Help?
Users often search for an EML to PST converter free when they have EML files that need to be opened in Outlook.
Free tools may be useful for small files or testing. However, they can have limitations. These may include file-count restrictions, limited export options, attachment problems, or incomplete folder conversion.
Before converting thousands of emails, test the software with a small copy of your data. Check the resulting PST carefully.
How to Verify the Recovered PST
After conversion or repair, do not immediately delete the original files.
Instead, verify the new PST.
Check these items:
Inbox and Sent Items
Custom folders
Email dates
Sender and recipient information
Attachments
Embedded images
Email formatting
Read and unread status
Folder hierarchy
Number of messages
Open several old emails from different folders. Also test emails containing attachments.
This simple verification step can prevent unpleasant surprises later.
Avoid Duplicate Emails During Migration
Duplicate emails are another common migration problem.
Microsoft 365 uses SourceEntryId-based matching when handling repeated imports of the same PST into the same target folder. However, importing different PST files with overlapping content can still result in duplicates.
For this reason, keep track of which PST has already been imported.
Do not repeatedly import different copies of the same mailbox without checking the target folder first.
Quick PST Migration Troubleshooting Checklist
Before trying the migration again, follow this checklist:
Keep a backup of the original PST.
Check whether Outlook can open the file.
Run ScanPST if corruption is suspected.
Check the PST file size.
Look for oversized emails.
Verify the folder hierarchy.
Use a Unicode PST when appropriate.
Test a small import first.
Check attachments after migration.
Verify the number of imported emails.
Watch for duplicate messages.
Keep the original data until the migration is confirmed.
Frequently Asked Questions
Why does Outlook say the PST file cannot be imported?
The PST may be corrupted, incompatible, too large, or contain problematic items. Start by creating a backup and running ScanPST. If the problem continues, test the PST on another Outlook installation.
Why are some emails missing after PST migration?
Some messages may have been corrupted or exceeded the supported message size. Microsoft 365 can skip individual items larger than 150 MB during PST import.
Can I repair a corrupted PST before importing it?
Yes. Microsoft's ScanPST.exe can be used to diagnose and repair certain PST problems. After repair, verify the data before starting another migration.
Is it better to split a large PST?
Yes, splitting a very large PST can make migration easier. Microsoft recommends keeping uploaded PST files at 20 GB or less for better import performance.
What should I do if EML files fail to convert to PST?
First, check whether the original EML files open correctly. If they are healthy, try a dedicated EML conversion solution and test the output with a small batch before converting the complete dataset.
Conclusion
Outlook import errors after PST migration can happen for many reasons. Corruption, large files, oversized messages, compatibility issues, and duplicate data are some of the most common causes.
The safest approach is to work step by step. Keep a backup, repair the PST when necessary, check file size, verify important emails, and test the migrated data before deleting the original files.
With proper preparation and verification, you can reduce migration failures and make your Outlook email transfer much more reliable.
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