Overview
A mid-sized engineering firm with 218 employees decided to move from Microsoft Outlook to Mozilla Thunderbird to reduce software licensing costs and simplify email management. The migration was fairly straightforward until the IT department discovered that nearly 1,320 PST files, totaling 486 GB of archived email data collected over 11 years, were not directly accessible in the new email platform.
The organization required all the historical emails of every employee for legal reference, project documentation and customer communication. More importantly, the attachments, sender, timestamps and folder hierarchy had to stay the same.
The main difficulty was finding a reliable PST to EML Converter that could manage large Outlook archives without data corruption and without needing Outlook to be installed.
Challenges
The system administrator responsible for the migration, encountered a number of hurdles.
· Over 3.8 million archived emails had to be migrated.
· Employees needed to have access to historical communication without interruption.
· Email was archived in separate PST archives of 500 MB to 14 GB across multiple departments.
· Folder hierarchy had to be the same after migration.
· For compliance audits, e-mail attachments and metadata were required.
· The migration had to be done in 5 business days.
We looked at a number of different ways to do this and it was obvious that the old methods were too slow and not reliable enough for a migration of this size.
Importance
The archived emails included engineering drawings, legal approvals, supplier quotes, invoices and customer contracts from more than ten years ago.
Any missing email could have an impact on current projects or future audits. Therefore, maintaining the integrity of the entire mailbox was more important than just changing file formats.
Manual Methods
Manual Method 1: Drag and Drop Export in Outlook
The IT team brought the PST files into Outlook and manually dragged emails into folders that could later be individually accessed.
Outcome
Small mailboxes exported successfully. Processing 1,320 PST files was very time-consuming.
Limitations
· Microsoft Outlook is required for all migration workstations.
· Folder hierarchy usually needed to be rebuilt manually.
· Mailbox size correlated positively with human error.
· Average processing speed was only 2 PST files per hour.
Manual Method 2: Forwarding E-mails to the New Mailbox
Another approach was to forward important emails from Outlook to Thunderbird accounts.
Outcome
Only recent communication would be subject to migration.
Limitations
· Changed original metadata.
· Read/Unread status is dead.
· Sometimes attachments were not transferred.
· Can’t migrate millions of archived emails.
Manual Method 3: Save Emails from Outlook EML individually
Technicians attempted to recover emails individually using the Save As function in Outlook.
Outcome
That worked for small folders, but it was unworkable for enterprise archives.
Limitations
· Repetitive steps.
· Manual recreation of folders
· Hundreds of labor hours required.
· Emails that will probably be skipped.
Software for PST to EML Conversion
To overcome these problems, the IT department decided to use WholeClear PST to EML Converter after testing on twenty departmental mailboxes.
The pilot test used the software to convert 42 PST files (18.4GB) in 39 minutes with the full folder structure and each attachment preserved.
All 1,320 PST files were processed over four working days after successful validation.
Result Based on Tools
· Successfully migrated 486 GB
· 3.8 million emails migrated
· 100% attachment retention
· 100% folder structure preserved
· Metadata was not changed
· No corrupted EML files detected
· Migration finished 18 hours ahead of deadline
Advantages
Batch Conversion: Processing hundreds of PST files simultaneously drastically cut down the migration time.
Preserve Folder Structure: Employees immediately knew the mailbox organization after migration.
Retains Email Metadata: Sender information, timestamps, CC, BCC and subject lines were preserved.
Attachment Integrity Complete: Every document, image, spreadsheet and PDF was accessible post-conversion.
No dependence on Outlook: Migration workstations converted without needing Microsoft Outlook to be installed.
Supports Large PST Files: Converted archives above 10 GB with no visible performance degradation.
Migration Faster: The overall duration of the project was reduced from an estimated 19 days to only 4 days.
Compatible with Most Email Clients: Converted EML files opened successfully in Thunderbird, Windows Live Mail and eM Client.
Disadvantages
Need for Initial Storage: Conversion of EML files during migration required temporary additional disk space.
Conversion Time Is Dependent On File Size: Of course, very large PST files took more time to process than smaller archives.
Required Basic Technical Knowledge: Administrators should understand mailbox organization before doing enterprise-scale migrations.
Steps in Tool-Based Method
1. Download the software to a Windows computer.
2. Start the application.
3. Choose and browse the PST file.
4. Preview of Mailbox Folder.
5. Select EML as the output format.
6. Choose the destination folder.
7. Use optional filters, if needed.
8. Begin the conversion process.
9. Validate Generated EML Files.
10. Import converted emails to the preferred email client.
Expert Recommendations
Tip | Benefit |
Backup PSTs prior to migration | Prevents unintentional data loss |
First test with a single mailbox | Verifies expected result |
Store output to SSD storage | Speeds up the conversion process |
Converted emails random check | Guarantees accuracy of migration |
Leave enough free disk space | Prevents broken conversion |
Organize output by department | Simplifies management going forward |
Don’t run heavy applications at the same time | Enhances processing performance |
Validate attachments post-migration. | Data preservation: confirmed complete |
Target Audience
This solution is perfect for:
· IT administrators in enterprises
· Managed Service Providers
· Government organizations
· Educational institutions
· Health care organizations
· Attorneys
· Financial advisors
· Companies moving away from Outlook
They enjoy less migration time with full mailbox integrity preserved.
The solution is very beneficial for Outlook migration, email archiving, platform replacement, compliance projects and legacy mailbox accessibility.
FAQs
Why did this project fail manual migration?
The organization had more than 3.8 million emails and manual methods were too slow and error-prone.
Was the email attachment saved?
Yes. All attachments were still available after conversion."
Was Outlook required for the migration?
No. The migration was carried out without using Outlook for the conversion.
Were there email folders?
Yes. The initial mailbox hierarchy was maintained during the migration.
Which email clients were able to open the converted files?
Converted emails were successfully imported into Mozilla Thunderbird, Windows Live Mail and eM Client.
How long did the whole migration take?
The whole project was completed in four working days, much ahead of the planned schedule.
Conclusion
This case study illustrates how a particular migration solution successfully addressed large scale email client compatibility challenges for 218 users, 486 GB of mailbox data and 3.8 million archived emails. The automated workflow offered faster execution, maintained mailbox integrity, retained attachments, maintained metadata and removed unnecessary administrative effort compared to manual approaches.
I recommend this solution for organizations, IT administrators, managed service providers and businesses planning for large Outlook migration projects where accuracy and reliability are important.
