OL97: Purpose and Capacity of Outlook Storage Facilities

This article discusses the purpose and capacity of each of the following storage facilities used by Microsoft Outlook 97 when running as a client to a Microsoft Exchange Server:

• Personal Folders (PST) files located on the user's computer.
• Offline Store (OST) files located on the user's computer.
• Mailbox Folders located on the Microsoft Exchange Server.
• Public Folders (PF) located on the Microsoft Exchange Server.

Personal Folders Files Located on Your Computer

One or more .pst files can reside on each user's computer. A .pst can be defined as your primary store or as a secondary store. A primary store means that mail delivered to the Inbox in that file and all of your other default Outlook folders, such as Calendar, Contacts, Deleted Items, Journal, Notes, Sent Items, and Tasks reside there. A .pst file can also be defined as a secondary store, which means you can move items to that file, either manually or with Inbox Rules, from your primary store.

Each .pst file can contain a maximum of 65,535 top level items, and a maximum of 65,535 items in each folder. An item is either a folder, message, task, etc. Upon reaching 16,384 items, you are upgraded to the a large tables format, which increases the limit from 16,384 to 65,535. The total file size of a .pst cannot exceed 2 Gigabytes (GB).

Offline Store (OST) Files Located on the User's Computer

Only one OST file can reside on each user's computer. The OST is always defined as your primary store, meaning that your mail is delivered to the Inbox in that file and all of your other default Outlook folders, such as Calendar, Contacts, Deleted Items, Journal, Notes, Sent Items, and Tasks reside there. The OST differs from the PST in that items residing in it can also reside in the user's Mailbox on the Server. You can synchronize the OST and Mailbox folders so that they always contain the same information. This allows you to work offline when the server is unavailable; then, when reconnected to the server, changes made offline upload to the server and mail received while offline downloads to the OST.

The OST file can contain as many folders as you desire and each folder can contain a maximum of 16,384 (16 kilobytes) records. The total file size of an OST cannot exceed 2 GB.

Mailbox Folders Located on the Microsoft Exchange Server

A Mailbox Folder for each user is stored on the server in the same database store. The server has a capacity of 16 GB for one store, which translates to millions of records. However, the server administrator may limit the size of each user's Mailbox Folder.

Public Folders (PF) Located on the Microsoft Exchange Server

Public Folders reside on the server in the same database store. They contain information shared by many users. Depending upon access permissions, individual users can read, update, or add to the PF. The server has a capacity of 16 GB for one store, which translates to millions of records.

For more information about this subject, please see the following articles in the Microsoft Knowledge Base: