Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteOutlook file extensions identify different kinds of data: PST and OLM can hold collections of Outlook items, OST is a synchronized offline copy, and EML and MSG hold individual messages. OFT is a message template, while ICS, VCF, and CSV are used to exchange calendar, contact, or tabular data. What a particular Outlook version can open or import depends on the file type and app edition.
What the common Outlook file extensions mean
| Extension | What it contains | Typical use |
|---|---|---|
| .pst | An Outlook Data File that can contain messages, contacts, appointments, tasks, notes, and journal entries. | Local storage, archiving, exporting, or backup. Support and steps depend on Outlook version and account requirements. |
| .ost | An offline synchronized copy of mailbox data for some Exchange, IMAP, and Outlook.com account configurations. | Working offline with account data. It is associated with the account and server copy, not a portable archive to move between computers. |
| .olm | An Outlook for Mac archive that can contain messages, contacts, calendar items, tasks, and notes. | Archiving or transferring Outlook for Mac data. It is not simply interchangeable with a Windows PST file. |
| .eml | An individual saved email message. | Saving or sharing a single message; Outlook and other email applications support the format. |
| .msg | An individual Outlook message. | Saving or sharing one Outlook message. Opening it in new Outlook has a Microsoft 365 subscription requirement. |
| .oft | An Outlook message template. | Reusing a message layout or content; it is not a mailbox archive. Opening it in new Outlook has a Microsoft 365 subscription requirement. |
| .ics | Calendar event or calendar data in iCalendar format. | Importing events or subscribing to an online calendar. An imported file is a snapshot; a subscription can receive later updates. |
| .vcf | Contact details in vCard format. | Sharing a contact and adding a received contact to Outlook. |
| .csv | Tabular data in comma-separated values format. | Contact import and export workflows; it is not a complete Outlook mailbox archive. |
How PST, OST, and OLM differ
PST: an archive or local data file
A PST can store a collection of Outlook items, so it is commonly used for local storage, archiving, export, or backup. Microsoft documents PST support in new Outlook with requirements, including a Microsoft 365 subscription for the account, and notes an ARM limitation. Its new Outlook and classic Outlook workflows differ, so check the instructions for the edition you use before opening or importing one: Microsoft’s PST guidance for new Outlook and guidance for classic Outlook.
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OST: a synchronized offline copy
An OST supports offline access to synchronized data for supported account configurations. It remains associated with the account and its server copy. Microsoft says the file can be recreated when the account is set up again, so copying an OST to another computer is not the normal way to move a mailbox. See Microsoft’s explanation of Outlook data files.
OLM: an Outlook for Mac archive
OLM is the archive format used by Outlook for Mac. Microsoft describes it as a data file for archiving email messages, contacts, calendar items, tasks, and notes. Microsoft also documents importing a Windows Outlook PST into Outlook for Mac; that cross-platform workflow does not make PST and OLM the same format. The cited Microsoft page states that support for legacy Outlook for Mac ends in October 2026, so consult current Mac-specific instructions if you are planning a migration: Microsoft’s Outlook for Mac archive and import guidance.
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Which files contain a message, template, calendar, or contact?
EML and MSG: individual messages
EML and MSG are saved-message formats, not collections of an entire mailbox. Microsoft documents opening EML and MSG files in new Outlook, with a qualifying Microsoft 365 subscription required for MSG: Microsoft’s instructions for EML, MSG, and OFT.
OFT: a reusable message template
An OFT file is a template for creating a message, rather than a saved mailbox or message archive. Microsoft lists a qualifying Microsoft 365 subscription as a requirement to open OFT files in new Outlook. Classic Outlook and new Outlook may have different workflows; use the instructions for the app you have.
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ICS: calendar events or an online calendar
An ICS file can be imported to add calendar events. Importing it gives you a snapshot; if you need later changes from the calendar owner to appear, subscribing to the online calendar is different and can receive updates. Available steps vary by Outlook edition. See Microsoft’s calendar import and subscription guidance.
VCF and CSV: contact data
A VCF file shares a vCard contact, which can be added to Outlook. CSV is used for tabular contact import or export, not for preserving a full mailbox with its folders and other Outlook items. Microsoft documents adding a contact from a vCard at its vCard guidance and contact transfer using CSV at its contact import instructions.
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How to identify the right file for your task
- Need a collection of Outlook items for storage or backup? Look at PST for Windows Outlook workflows or OLM for Outlook for Mac workflows; check current version requirements before importing.
- Need offline access to an account mailbox? OST is the synchronized local copy, not a portable archive.
- Need to save or open one email? The file may be EML or MSG.
- Need a reusable message? OFT is the template format.
- Need to move calendar or contact details? ICS is for calendar data, VCF for a vCard contact, and CSV for tabular contact transfer.
Can renaming an extension convert an Outlook file?
No. Changing a filename from .pst to .ost, for example, changes only the label, not the contents or format. Use an Outlook-supported import, export, or conversion workflow appropriate to the file and app instead. Microsoft’s file-extension guidance explains the distinction: Microsoft’s explanation of file extensions.
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