The Problem: Large PST Files Create Conversion Challenges
PST files grow over years of daily email use. A typical business user might accumulate 5-10 GB of email in a few years, and power users or shared mailbox archives can easily reach 20, 30, or even 50 GB. Converting these large files brings specific challenges that smaller files do not face.
The main issues are upload time (sending a 20 GB file over a standard internet connection takes time), processing time (parsing millions of messages requires significant server resources), output size (the converted files need to be downloaded and stored), and stability (network interruptions during long uploads can force you to start over).
This guide covers practical strategies for handling large PST conversions efficiently.
What Counts as a “Large” PST File
| File Size | Message Count (Approx.) | Typical User |
|---|---|---|
| 1-5 GB | 10,000-50,000 | Standard individual user |
| 5-10 GB | 50,000-100,000 | Power user, several years of email |
| 10-20 GB | 100,000-200,000 | Shared mailbox, department archive |
| 20-50 GB | 200,000-500,000+ | Enterprise archive, legal hold |
Solution: Convert Large PST Files With MailtoPst
MailtoPst supports PST files up to 50 GB on the Enterprise plan, making it one of the few online tools capable of handling enterprise-scale email archives.
Preparation Steps
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Check your internet connection — for a 10 GB file, you need a stable connection. A 50 Mbps upload speed transfers 10 GB in roughly 25 minutes. A 10 Mbps connection takes about 2 hours. Use a wired connection for reliability.
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Verify the PST is healthy — run ScanPST.exe on the file before converting. Large files are more prone to minor corruption that can slow down or complicate conversion. See How to convert a corrupted PST.
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Check available disk space — the converted output will be roughly the same size as the input. Converting a 10 GB PST to EML produces approximately 10 GB of EML files in a ZIP archive. Make sure you have enough space for both the source file and the downloaded output.
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Close Outlook — if the PST is currently open in Outlook, close Outlook before uploading to avoid file locking issues.
Conversion Process
- Go to mailtopst.com and select your target format.
- Upload the large PST file. A progress bar shows the upload status.
- Processing begins automatically after upload. Large files may take 10-30 minutes to process depending on message count and complexity.
- Download the result when processing completes.
For very large files, you can close the browser tab after upload and return later — the conversion continues on the server.
Alternative: Split Before Converting
If your PST file is extremely large or your internet connection is limited, splitting the file into smaller pieces before converting can be more practical.
How to Split a PST in Outlook
- Open the large PST file in Outlook.
- Create a new empty PST file: File, then Account Settings, then Data Files, then Add.
- Select a subset of messages (e.g., all emails from 2020-2022) and move them to the new PST.
- Repeat for other date ranges or folders.
- Convert each smaller PST file separately.
Split by Folder
Alternatively, export specific folders from the large PST. Go to File, then Open & Export, then Import/Export, then Export to a file, and select individual folders rather than the entire PST.
Choosing Your Plan
| Plan | Max File Size | Best For |
|---|---|---|
| Free | 100 MB | Testing with a sample of your archive |
| Standard | 50 GB | Individual users and small teams |
| Enterprise | 50 GB (priority) | Organizations, legal teams, bulk migrations |
The Enterprise plan includes priority processing, which means your large file conversion is queued ahead of standard-tier jobs.
Tips for Large File Conversion
- Convert to EML if possible — EML output gives you individual files, making the download a ZIP that can be extracted incrementally. MBOX produces a single large file that must be downloaded completely before use.
- Schedule for off-peak hours — upload during nights or weekends when your internet bandwidth is less contested.
- Keep the original — always retain the original PST file until you have verified the converted output is complete and correct.
- Monitor the output size — a 20 GB PST converted to EML produces roughly 20 GB of files (plus some compression in the ZIP). Plan your storage accordingly.
Why MailtoPst for Large Files
- 50 GB capacity — one of the highest limits available for online email conversion.
- Server-side processing — the heavy work happens on our servers, not your local machine. No RAM or CPU limitations on your end.
- Corruption tolerance — even large files with minor issues are processed reliably. The engine skips damaged blocks and extracts everything recoverable.
- Secure handling — TLS encryption, EU servers, auto-deletion. GDPR compliant regardless of file size.
- Background processing — no need to keep the browser open during conversion.
Upload your file now and tackle your large PST conversion. Try free — no credit card required.
Frequently Asked Questions
What is the maximum PST file size MailtoPst supports?
The current maximum is 50 GB on the Standard and Enterprise plans. The free tier supports up to 100 MB for testing purposes.
How long does it take to convert a 10 GB PST file?
Upload time depends on your internet speed (roughly 15-60 minutes for 10 GB). Server-side processing typically takes 10-20 minutes for a 10 GB file. Total time: 30-80 minutes.
Can I convert a PST file larger than 50 GB?
Files over 50 GB should be split into smaller pieces using Outlook before converting. See the splitting instructions above. The Enterprise plan can handle the largest files up to the 50 GB limit.
Will the conversion time out for very large files?
No. MailtoPst’s server-side processing does not time out. The conversion runs until completion regardless of file size. The only timeout risk is during the upload phase — use a stable connection.
Is it faster to split the PST or convert it whole?
For files under 20 GB, converting the whole file is typically faster. For files above 20 GB, splitting by date range and converting in parallel can save total time, especially if your upload speed is limited.
Upload your file now and convert your large PST archive with confidence.