ANSI or Unicode PST: how to tell, and what opens each
A .pst made before Outlook 2003 is ANSI and has a 2 GB limit. Outlook 2003 and later write Unicode files. The two formats differ in a version field in the file header, so you can tell without opening Outlook.
| File | Bytes 0-3 | Version at byte 10 | Format | Mailattic result |
|---|---|---|---|---|
| small.pst (32 emails) | 21 42 44 4E (“!BDN”) | 23 | Unicode | Opens: 6 folders listed in 9 ms, 32 emails exported |
| ansi-fake.pst (same file with version 14) | 21 42 44 4E (“!BDN”) | 14 | ANSI | Refused with an “unsupported format” message, nothing exported |
Details
Every .pst starts with the same four bytes. The version number decides the format: 14 or 15 is ANSI, 23 or higher is Unicode. If your file is ANSI, open it once in Outlook and use File > Open & Export > Import/Export to save it as a new Unicode .pst, then open that one.
The ANSI row is a Unicode test file whose version field was changed to 14 to check that the program refuses it clearly instead of showing garbage. It is not a real Outlook 97 archive. Mailattic reads Unicode PST only. Tested 2026-10-31. mailattic-core 1.0.0.
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About Mailattic
Mailattic is our Windows app that opens a .pst without Outlook, reads attachments, and exports EML, MBOX and PDF. Read-only: your archive is never changed.
Not affiliated with, endorsed by or connected to Microsoft. Outlook and Microsoft are trademarks of Microsoft Corporation and are used only to say which files this opens.