Convert OST, OLM, MBOX, EML or MSG to PST

When you need a .pst file

• An orphaned .ost. Outlook only uses an .ost with the account and PC that made it. When the account is closed or the PC replaced, converting the .ost turns its mail into a .pst for Outlook.
• From a Mac to Windows. Outlook for Windows cannot read an Outlook for Mac .olm archive; converted, it becomes a .pst for Outlook for Windows.
• Out of Gmail or Thunderbird. Outlook cannot import an .mbox file. Google Takeout and Thunderbird export MBOX; converted, the mail becomes a .pst for Outlook, with its folders or Gmail labels.
• Loose messages. Hundreds of .eml or .msg files from a backup or a court production become one .pst mailbox file for Outlook.

What is kept, source by source

• .ost and .pst: every folder with its type, and every item with all its stored properties: bodies in every form (plain, HTML, Rich Text), headers, flags, categories, attachments and attached items, contact, calendar and task fields. Old ANSI .pst files (Outlook 97–2002) convert too, but have had less testing.
• .olm: mail folders with their messages and attachments, contacts, calendar events (with simple repeats), tasks and notes.
• .mbox: every message with its headers, text, HTML, attachments and inline images, in one folder or, for a Google Takeout file, in one folder per Gmail label.
• .eml and .msg: each message with its attachments; a dropped folder keeps its subfolders.

What is not kept

• Search folders, views and rules of an .ost or .pst; objects embedded in Rich Text messages (OLE objects) are left out, and the item says so.
• From an .olm: categories, flags, priority, free/busy status, contact photos, contact groups, meeting attendees, and repeats such as “the second Tuesday of every month”.
• From an .mbox: a message with several Gmail labels goes into one folder only; Gmail stars and categories are not kept.
• Password-protected .pst files and files with Outlook’s old “high encryption” cannot be opened.

How big the .pst can be

Chrome and Edge on a computer write the .pst straight into a file on your disk as it is made, up to 3 GB per file. Otherwise the file is built in the browser first: up to 2 GB, or 1 GB in Safari and on iPhone and iPad, which keep it in memory. One conversion takes up to 100,000 items, and an item larger than 64 MB is left out and listed.

When a mailbox is bigger, the conversion stops at the limit, keeps a complete .pst of what it converted and tells you how many items are left: convert the rest folder by folder into a second file. Outlook opens several .pst files side by side.

Opening the .pst in Outlook

• To look through it: File → Open & Export → Open Outlook Data File, then choose the .pst. It appears as its own mailbox in the folder pane.
• To copy its items into your mailbox: File → Open & Export → Import/Export → Import from another program or file → Outlook Data File (.pst).
• The file is written to Microsoft’s published specification for Unicode .pst files (Outlook 2003 and later), and our tests open converted files with two independent PST readers. Outlook itself is not part of those tests yet, so keep the original mailbox until you have looked through the .pst. If Outlook reports a problem, its Inbox Repair Tool (scanpst.exe) can check the file; please write to support@cantopen.com, because it is a bug we want to fix.

Why nothing is uploaded

Mailboxes are private and large. Desktop converters ask you to install software; online converters ask you to upload the whole mailbox. Here the conversion runs in your browser tab: the mailbox is read from your disk and the .pst is written back to your disk. No code on the page sends the file, its name or its contents anywhere.