Components of Public Folders in Exchange Server

There are three major components of a public folder in Microsoft® Exchange Server, and each one gives you some important functionality that you might need:

  • Public folder hierarchy object   This is an actual folder name object that you can see from either Exchange System Manager (under the Public Folders tree) and the Microsoft Office Outlook client. All of the public folder hierarchy objects make up the tree view that you browse. Hierarchy objects live in the Microsoft Exchange Information Store public folder database. Folder client permissions or a folder replica list are examples of data that are contained in the folder hierarchy.
  • Public folder content   This is the actual data that you can post, create, or mail into the public folder. The data will live permanently only on the public folder store that has a replica of a specific folder.
  • Public folder directory object   This is the Active Directory® directory service representation of the public folder that, among other things, contains the e-mail address of the folder. The public folder directory object lives under the Microsoft Exchange System Objects organizational unit (OU) under the domain, which you can view through the Active Directory Users and Computers Microsoft Management Console (MMC) snap-in. If you do not see this OU, select the View > Advanced Features menu in the Active Directory Users and Computers MMC snap-in.

Another important part of the client public folder experience is the actual Public Folders tree that you can expand to see the public folder hierarchy. The Public Folders object is an Active Directory object that lives under the Administrative Group's Folders object. There is only one MAPI public folder hierarchy that a single Exchange organization can support. This tree object is located in the administrative group that had Exchange 200x installed into it first. The Public Folders object has an attribute that points to every public folder store that is associated with it. You can move this specific object between administrative groups by just dragging and dropping it in Exchange System Manager.

What replicates all this stuff?

The hierarchy is replicated by the public folder replication process, which cannot be scheduled or limited only to some servers. Hierarchy replication is automatic to all servers that have public folder stores in the organization. Replication messages are mail-based and are sent from public folder store to public folder store.

The content is also replicated by the public folder replication process. However, this replication can be scheduled. By default, the content replication is set to happen "always" which really means "every 15 minutes." Replication of content happens only between the servers that have replicas for specific folders defined. The replica list for a specific folder is contained in the public folder hierarchy. There is no way to replicate the folder content to a server if the hierarchy for that folder did not replicate first.

Public folder directory objects are replicated by the Active Directory replication process. Because these objects live under the domain naming context, they are replicated only within the domain where they exist. However, the directory objects might not exist for folders at all. They are created by default if the Exchange organization is in mixed mode, but, when the organization is in native mode, newly created public folders will not have the directory object by default. Administrators must mail-enable folders first before their directory objects are created.

One more thing to remember is that even though public folder directory objects are domain-specific, if folders are mail-enabled, a subset of the folder's directory object attributes will be replicated to global catalog servers within the forest. That replication occurs because, if it is mail-enabled, a folder is a possible e-mail recipient. Therefore it needs to be "addressable" from any domain in the forest, so it can be addressed to or selected from the global address list (GAL), assuming that it is not hidden. 

The Public Folders tree itself is also replicated by the Active Directory replication. This object lives under the configuration naming context, and it is replicated between domains by default. For example, if the object is created by the first installed Exchange server in the root domain, a second Exchange server installed in the child domain later sees the object and therefore does not attempt to create its own. If the second Exchange server did not see the object, the organization might have two MAPI folder trees, which is a non-supported scenario with all sorts of problems.

What is the role of ADC in all this?

Something that people get confused about when they migrate from Microsoft Exchange Server 5.5 to Exchange 200x is the role of the Public Folder Connection Agreement (PFCA) in Active Directory Connector. They mistakenly believe that the PFCA has something to do with public folder replication between Exchange 5.5 and Exchange 200x. That is not the case at all.

PFCA replicates only public folder directory objects from the Exchange 5.5 directory to Active Directory and vice versa. Directory objects are mainly used when someone tries to mail a public folder. In other words, without PFCA, the Exchange 200x recipients will not be able to mail Exchange 5.5 public folders. The same goes in the other direction – if directory objects for Exchange 200x public folders are not replicated to Exchange 5.5, the Exchange 5.5 users would not be able to mail Exchange 200x folders either.

There is one more use for replicating public folder directory objects between Exchange 5.5 and Active Directory. In Exchange 5.5, in order to administer a public folder, the directory object for the folder has to be present in the 5.5 directory. Therefore, if Exchange 200x public folder directory objects are not replicated using the PFCA from Active Directory to Exchange 5.5 directory, attempts to administer the Exchange 200x public folders using the Exchange 5.5 Administrator tool result in popup errors and an inability to make any changes to the folder.

For More Information

To learn more about public folders in Exchange, see the following resources: