My initial guess is that you have one of these components installed that only work with Exchange 2000/2003:
Key Management Service
Mobile Information Server
Could it be that you are trying to install an Exchange 2003 server into your existing Exchange 5.5 implementation to continue on with an intra-organizational migration? If so, just ensure those components are removed prior to the install.
MEMBER FEEDBACK TO THIS ASK THE EXPERT Q&A:
The named error could also occur if the Exchange 5.5 server has its LDAP protocol turned off, if the default LDAP port number from 389 has been changed, or the Exchange 5.5 server is on a Windows 2000 domain controller resulting in an LDAP port conflict.
Some ExDeploy tools, such as the Active Directory Connector (ADC), will use this port in communicating with Exchange 5.5; you can change the LDAP port and the authentication method in the ExDeploy configuration setup.
It would seem an unintentional misstatement that the listed components only work with Exchange 2000/2003. The listed components work with Exchange 5.5 and Exchange 2000, but are obsolete in Exchange 2003. They can continue running in a mixed Exchange 5.5/2000/2003 organization as long as they are not on the Exchange 2003 server.
Also, it is not necessary to ensure the listed components are removed prior to the install of the Exchange 2003 server into the Exchange 5.5 site. Removal would be required if the user planned to upgrade an Exchange 2000 server to an Exchange 2003 server with these installed.
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This was first published in March 2006