When you use Outlook in Offline mode, you can work in Outlook as if you were connected to the server when you are not. Offline Folders allow you to compose email and make updates to items such as Contacts and Calendar events when you are unable to have a network or Internet connection, such as on an airplane. Once you are connected to a network again, Outlook will synchronize these changes with your mailbox on the Exchange Server, including sending messages that are waiting in the Outbox.

In order for Offline mode to work, Outlook creates an Offline Storage file (OST file) on your computer. When you first enable Offline Folders, all the information in your Exchange Server mailbox is copied to this file. The size of the OST file is proportional to the amount of data in your Exchange mailbox. This initial one-time process can take a long time for larger mailboxes.


  • Like Personal Folder files (.pst files), the .ost file can become corrupted and can be compacted to save space on your computer. Read the Knowledge Base article on How To Create, Compact, Repair Or Delete Offline Folders for more information.
  • Large mailboxes (hundreds of megabytes or gigabytes) with thousands of messages or other items are more likely to experience poor performance or corruption problems. Read the Knowledge Base articles on How To Export Data In Outlook and How Does Outlook Archiving Work? for more information on managing mailboxes. 
  • You cannot use an OST file that has previously been used with a different mailbox.
  • Extensive use of Offline Folders and offline synchronization may cause performance issues when using an Outlook client to connect to the Exchange server.
  • If you have Terminal Services installed on the Windows 2000 Server computer, you will not be able to install and use Offline Folders and the Offline address book.

Read the Knowledge Base article on How To Create, Compact, Repair Or Delete Offline Folders for more information.