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Open an Outlook OST file

An .ost file is Outlook’s offline copy of a Microsoft 365, Exchange or Outlook.com mailbox. Outlook only uses it together with the account it belongs to – you cannot simply open an .ost in Outlook as a data file. Letterlark can read it directly.

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Steps

  1. Find the .ost file (on Windows usually under %LOCALAPPDATA%\Microsoft\Outlook). Close Outlook first, or copy the file, because Outlook keeps it locked.
  2. Open Letterlark and open the .ost file.
  3. Browse folders and messages, search, and export what you need.

What to expect

  • Mail, attachments and folders that were synchronised to the computer are in the OST. Items that were only on the server may be missing.
  • Newer OST files use compressed blocks; Letterlark supports this format, but unusual or damaged files may still fail to open.
  • Only open OST files that you are entitled to access, for example your own old work profile with your employer’s permission.

Questions and answers

Can I convert OST to PST?

Letterlark does not write PST files. It exports to EML, MBOX, PDF and CSV, which other mail programs can import.

Does it work for orphaned OST files?

Usually yes – Letterlark does not need the Outlook profile or the server account. It reads the file contents directly.

Last updated: 2026-10-26