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Open an Outlook .ost file without Outlook

5 October 2026 · searched as open ost file without outlook

An .ost is the copy Outlook keeps of an Exchange or Microsoft 365 mailbox. Once the account is gone — a closed company mailbox, an old laptop — Outlook will not open it any more. MailCode reads it the way it reads a PST, and gives you its mail, folders and attachments to search.

How to do it

Copy the .ost somewhere you can reach it. On Windows, Outlook keeps it in %LOCALAPPDATA%\Microsoft\Outlook; close Outlook first if it is still using the file.
In MailCode, choose File ▸ Import PST… and click Add files…. The picker takes .ost as well as .pst.
Name the archive folder. The suggested name keeps the .ost ending; change it to whatever you will look for later.
Leave Keep the PST's folder structure ticked, check the list of folders it will write, and click Import →.

MailCode: An .ost in Import PST…, its folders listed before anything is written. — An .ost in Import PST…, its folders listed before anything is written.

An .ost keeps its mail one level deeper than a PST, so the folders arrive under IPM_SUBTREE — Inbox, Sent Items and Calendar sit there. You get what Outlook had downloaded: an .ost set to keep the last year holds the last year. A file Outlook encrypted is refused with PST is encrypted, and one that is damaged or not an Outlook data file reads Invalid file header. Outlook for Mac does not use .ost files; it exports .olm, which MailCode does not read.

MailCode is a mail client for Mac and Windows that keeps your mail in a searchable archive on your own disk. Free for 14 days.

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