Yes, there are tools to synchronize between two separate Exchange Server organizations, even of different versions.
There are two versions of Microsoft's Identity Integration Server (MIIS). Let's start with the basic and free version. The MIIS Identity Integration Feature Pack will let you synchronize between Active Directory (AD) in different Windows 2000 and Windows 2003 AD Forests. There is also the full version of MIIS which has a feature called Global Address List Synchronization for synchronizing with Exchange 5.5; it also works with other directories and can be customized.
For synchronizing free/busy information, you can use a tool called the InterOrg Replication Utility. This tool will also allow you to synchronize public folders between the Exchange organizations.
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This was first published in March 2004