LetterlarkPST file too large: limits and what to do
Outlook becomes slow or refuses a PST long before your disk is full. Here are the actual limits and the practical ways around them.
Updated 2026-10-26 · 5 min read
The limits
| PST type | Default limit |
|---|---|
| ANSI (Outlook 97–2002) | 2 GB (hard limit) |
| Unicode, Outlook 2003/2007 | 20 GB |
| Unicode, Outlook 2010 and later | 50 GB |
Administrators can raise the Unicode limit with the MaxLargeFileSize and WarnLargeFileSize registry values, but Microsoft warns that very large files hurt performance.
“File too large for the destination file system”
That's not Outlook – it's a USB stick formatted as FAT32, which can't hold files over 4 GB. Reformat the drive as exFAT or NTFS (this erases it), or copy via a network drive.
Outlook won't open a huge PST
- Run the Inbox Repair Tool (scanpst.exe) – large files are more often damaged.
- Open it in a fresh Outlook profile.
- Split it with Outlook's Archive feature or by moving folders into a new PST.
Read a large PST without Outlook
Letterlark reads PST files in chunks straight from disk, so a 20–50 GB archive does not have to fit in memory. Opening still takes time because every message is indexed for search. You can then export folders to MBOX/EML or save single messages – exporting folder by folder avoids memory limits on very large exports.
Open your mail file now. Letterlark opens PST, OST, MSG, EML and MBOX files in Chrome, Edge, Firefox or Safari on Windows, Mac, Linux or Chromebook. Files are read on your computer – the app page cannot upload anything. Viewing and search are free without limit; exports (PDF, EML, MBOX, CSV, attachments) are free up to 20 messages at a time. A one-time license is $19 (Personal) or $49 (Pro, business use).
Questions and answers
Can I split a PST without Outlook?
Letterlark doesn't write PST files. Export folders to MBOX or EML instead, or use Outlook's archive/move features.
Is a 50 GB PST safe?
It works, but backups, sync and repair get slower and riskier. Splitting by year is a common practice.
Sources and product names: Microsoft Learn: "How to configure the size limit for both (.pst) and (.ost) files in Outlook"; Microsoft guidance on FAT32 4 GB file limit. Microsoft, Outlook, Gmail, Google Takeout, Apple Mail and Thunderbird are trademarks of their owners. Letterlark is independent and not affiliated with them.