Exchange 2003.Event ID 8026 MSExchangeAL LDAP Bind was unsuccessful on directory for distinguished name. Directory returned error: [0x51] Server Down.

You demote your domain controller and then your exchange starts logging the following event.

Source: MSExchangeAL
Category: LDAP Operations
Event ID: 8026
User: N/A
Computer: "name of your exchange server"
Description: LDAP Bind was unsuccessful on directory "name-of-demoted-domain-controller" for distinguised name. Directory returned error: [0x51] Server Down.

Cause:
Your recipient update service was set to work with the demoted domain controller. Since the domain controller has been demoted, the recipient update service will shout when it doesn't find what its looking for.
Recipient Update service plays a role in creating exchange specific attributes in active directory like automatic generation of SMTP addresses that has been defined for users. You can read more of this in http://support.microsoft.com/kb/319065.

Solution:

1. Open the exchange "System Manager".
2. Expand Recipients container and then click on "Recipient Update Services".
   
   
   
   3. On the right panel, double click each of the recipient update services and change the "Windows Domain Controller" to point to your new windows domain controller.