Selecting the Appropriate Client for Exchange Public Folder Access
Generally, access to Microsoft® Exchange 2000 Server and Microsoft Exchange Server 2003 public folders is integrated with regular mailbox access through the following protocols:
Microsoft Office Outlook® 2003 (MAPI clients)
Clients that are compatible with Internet Message Access Protocol (IMAP), such as Outlook Express
Microsoft Office Outlook Web Access (Web client)
Offline access to public folder content is available through the Favorites public folder in Outlook in the Favorites public folder.
Top-Level Hierarchies for Public Folders
Before you can determine which client is best suited for your public folder needs, you should understand the difference between the two top-level hierarchies for public folders.
The top-level hierarchy (TLH) is the root of a public folder tree under which sub-folders are created. In Exchange Server 5.5, there was only one TLH. In Exchange 2000 Server and Exchange Server 2003, you can create multiple TLHs.
There are two types of TLHs in Exchange 2000 Server and Exchange Server 2003:
The MAPI TLH is the default public folder hierarchy that is created when Exchange Server 5.5, Exchange 2000 Server, or Exchange Server 2003 is installed. There can only be one MAPI TLH in an Exchange Server organization.
The application TLH was introduced in Exchange 2000 Server to allow multiple TLHs. All new non-default TLHs that an administrator can create are referred to as application TLHs.
From the perspective of client access, each TLH type has its own limitations. MAPI clients, such as Outlook, can access the MAPI TLH, which is called Public Folders in the List view of the Outlook client, but MAPI clients cannot access the application TLH. The same limitation applies to IMAP clients, such as Outlook Express. However, only the MAPI TLH can be viewed. POP3 clients cannot view public folders. However, Outlook Web Access can view both the MAPI TLH and the application TLH. The MAPI TLH is shown in Outlook Web Access when the user clicks the Public Folder link. To access application TLHs from Outlook Web Access, a user must enter the URL to the public folder virtual directory as in the following example:
https://mail.contoso.com/public/virtual\_directory\_name
Optimizing the Client Access Point
The following table summarizes the client access functionality that is described earlier.
| Client | MAPI TLH | Application TLH |
|---|---|---|
MAPI (Outlook) |
Yes |
No |
IMAP4 |
Yes |
No |
POP3 |
No |
No |
Outlook Web Access |
Yes |
Yes |
After you have decided which type of client access that you will deploy, you can optimize the client access point of your public folder solution. For example, if you decide to support Outlook Web Access as a client, you can optimize the TMP drive on the HTTP servers that service the Outlook Web Access requests.
Additionally, your choice of client determines in which type of database the data is stored. There are two options: rich-text file (.edb files) or streaming native Internet content (.stm files). Your configuration of the hardware for these stores affects performance and scalability.
For more information about optimizing for public folder access, see Optimizing Storage for Exchange Server 2003 and the Exchange Server 2003 Performance and Scalability Guide.
For More Information
To learn more about public folders in Exchange Server, see the following resources: