“Outlook data types” can refer to two different things: files that store or synchronize mailbox data, and files that represent individual messages, events, contacts, or other items. The key distinction is that PST and OST are data-store files, while MSG, ICS, VCF, and CSV are used for individual items or contact data exchange.
Outlook data-store files: PST and OST
PST and OST are both Outlook data files, but they serve different purposes. A PST can hold a folder hierarchy of messages and attachments and is commonly used for archives, exports, or POP account storage. An OST is an offline copy that synchronizes with server information for supported account types. It is not a portable archive.
| Format | What it represents | Typical role | What to know |
|---|---|---|---|
| PST | A message store containing folders, messages, and attachments | POP account storage, archives, or exports | Useful when you need a data file to retain or transfer. Microsoft’s PST specification defines its folder-and-message structure: [MS-PST]: Outlook Personal Folders (.pst) File Format. |
| OST | An offline Outlook data file | A synchronized copy of server information for certain account types | It depends on the account’s server synchronization; it is not a portable archive. Microsoft says an OST can be synchronized again from the server and does not need backup. |
Do not assume every Outlook account uses the same file arrangement. Microsoft’s guidance distinguishes account types and applies to Outlook 365, Outlook 2024, 2021, 2019, and 2016; the file used depends on account setup and product. See Microsoft’s instructions for creating an Outlook data file for the applicable account details.
Individual Outlook item and interchange formats
These formats do not represent a whole mailbox in the way a PST does. They represent one item or structured contact data, making them useful when saving or moving specific information.
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| Format | What it represents | Typical use |
|---|---|---|
| MSG | An individual Outlook item | Saving an email, appointment, contact, task, or similar item as a file. Microsoft’s [MS-OXMSG] Outlook Item (.msg) specification describes MSG as a format for individual Outlook items. |
| ICS | A calendar item | Calendar event interchange. |
| VCF | A contact item | Contact interchange or import. |
| CSV | Tabular contact data | Contact import or export; it is not a mailbox container like PST. |
Microsoft’s Outlook import and export guidance covers PST transfers and CSV or VCF contact workflows. For software that needs to identify Outlook item extensions, Microsoft’s MsoFileType enumeration maps item types to MSG, ICS, and VCF.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which format should you use?
- To retain or export a mailbox: consider PST, then follow the steps for your account type and Outlook version.
- To work offline with server-backed mail: OST is a synchronized local copy, not a substitute for an independent portable archive.
- To save one Outlook item: use MSG for supported individual items.
- To exchange a calendar event or contact: ICS is for calendar items; VCF is for contacts. CSV is another contact-transfer option when tabular data is appropriate.
Choose based on both the information you need to move and the account arrangement in Outlook. The formats are not interchangeable: a contact CSV does not contain a mailbox, and an OST is not an archive designed to move between accounts.
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