While mixed Exchange 2003/2007/2010 environments are supported, the requirements of the Exchange edition running on the Add2Exchange target server must be followed. The target server is the server which Add2Exchange has been configured to synchronize with during the install, which is shown in the registry key:
HKEY_LOCAL_MACHINE\Software\OpenDoor Software®\Add2Exchange\Server
Note that it is the Server value, not the DBServer value.
Because of the limitations of CDO, Add2Exchange may not be installed on an Exchange 2003 server in order to synchronize with an Exchange 2007 or 2010 server. If you need to synchronize with Exchange 2007 or 2010, you must install Add2Exchange where you can install Microsoft's standalone CDO. You cannot install it on an Exchange 2003 server because Exchange 2003 already uses its own version of CDO, which cannot be modified.