Offline folders are a feature of Microsoft Exchange Server that makes public folders and mailbox messages available to users when they work offline. Folders configured for offline availability are stored in a file with the .OST extension on the user's disk. When users log on to their home server and the network connection is reestablished, Microsoft Exchange Server automatically synchronizes the offline copy of the folder that it stored locally on the client computer with the copy on the server computer. This type of synchronization is known as local replication.
If a user has configured an offline folder identical copies of this folder exist in the information store on the server and in the OST file. When an application writes to this folder, it does not specify — nor need to know — whether the folder is currently online or offline. If the application is online, it writes to the online (server) copy of the folder. If the application is offline, it writes to the offline (OST) copy. This mechanism is automatic and is controlled by the information store.
When the offline information store is synchronized, its entire hierarchy table (including the user’s Favorites folders and the user’s private information store) is synchronized. Folders not designated as offline folders are also synchronized, but those folders cannot be written to and have an empty contents table when the computer is offline.
If both the offline information store and the information store on the server have changed at the time they are synchronized, changed data on the OST is first copied to the server, and then changed data on the server is copied to the OST. If this is an automatic synchronization — that is, it is occurring because the user has reconnected to the server — data is copied in only one direction from the OST to the server, regardless of whether data on the server has changed.
If an information store is offline, it contains the PR_STORE_OFFLINE property, which is set to TRUE.
An offline folder is permanently available if it was created while the computer was offline. If the folder was created while the computer was online, it will be available offline only if the user designates it as an offline folder.
For more information, see Offline Folders and RTF-Awareness.