  Exchange&nbsp;5.5 and Earlier Earlier versions of Exchange used directory databases. The directory database contains configuration information as well as distribution lists, custom recipients, and some mailbox information (the directory stores only the mailbox directory object; the Information Store database stores the actual mailbox content). The directory also stores public folder directory objects, which are by default hidden directory objects in the Recipients container. Exchange&nbsp;2000 and Later Exchange&nbsp;2000 and Exchange Server&nbsp;2003 do not have their own directory. Instead, both Exchange&nbsp;2000 and Exchange&nbsp;2003 use Active Directory. The actual mailbox and public folder information is still stored in the Exchange store. Communicating Between Versions Coexistence and migration can be a challenge because Exchange&nbsp;5.5: Predates Active Directory, and Cannot communicate with servers running Exchange&nbsp;2000 and later as easily as it can with other servers running Exchange&nbsp;5.5. So, the first server running Exchange&nbsp;2000 or Exchange&nbsp;2003 that joins each Exchange&nbsp;5.5 site creates and runs a special database called a Site Replication Service (SRS). Site Replication Service SRS is essentially an Exchange&nbsp;5.5–style directory that runs on Exchange&nbsp;2000 or later servers and really gives Exchange&nbsp;5.5 a directory "to talk to" on the Exchange&nbsp;2000 or later side. SRS does not have some of the limitations that the Exchange&nbsp;5.5 directory has, so it plays a crucial part in Exchange&nbsp;5.5 and Exchange&nbsp;2000 and later coexistence and migration situations. Essentially, configuration changes to the Exchange organization that are made in Active Directory replicate from the Active Directory: Into the SRS database though the ADC connection agreement, and then, From the SRS into the Exchange&nbsp;5.5 directory through the standard Exchange&nbsp;5.5 directory replication process.
Active Directory Connector When you migrate, you need some way to move your directory objects from the Exchange&nbsp;5.5 directory into Active Directory. Active Directory Connector (ADC) is the solution. The ADC is what will synchronize the Exchange&nbsp;5.5 directory with Active Directory. This is done through the use of Connection Agreements. There are three types of Connection Agreements that the ADC for Exchange&nbsp;2000 and later supports: Recipient Connection Agreement (RCA), responsible for replication of mailboxes, distribution lists, and Custom Recipient directory objects from the Exchange&nbsp;5.5 directory to Active Directory and back.
Again, this replicates directory objects only, so this does not have anything to do with the moving of actual mailbox content. Public Folder Connection Agreement (PFCA), responsible for replication of public folder directory objects between Exchange&nbsp;5.5 and Active Directory. Public folder directory objects are used when sending an e-mail message to a public folder and when administering a folder, especially on Exchange&nbsp;5.5.
So again, this is not for replication of actual public folder content between servers. Configuration Connection Agreement (Config_CA), responsible for replication of organization and site configuration information such as servers and connectors. Note: Always use the most recent version of the Exchange ADC and not a Microsoft Windows® ADC, which was released on the Windows&nbsp;2000 CD, because the Windows&nbsp;2000 ADC is not capable of doing many things that the Exchange&nbsp;2000 ADC can do and it has not been updated since its release. &nbsp; 
