The issue occurs when you are trying to create mailbox for new user using ADUC in Exchange 2003
By Microsoft the reason for the issue is : After upgrade of the forest to a 2003 native mode functional level, the Recipient Update Service may overwrite the value of the homeMDB attribute for new Microsoft Exchange Server 2003 users.

To fix the issue:

1. Open ADSI Edit.
2. Double-click the Configuration container
3. Expand CN=Services
4. Expand CN=Microsoft Exchange,
5. Expand CN=<Your ExchangeOrganizationName>.
6. Click CN=System Policies.
7. In the right pane, right-click CN=Mailbox Enable User, and then click Properties.
8. Scroll down to select the purportedSearch attribute, and then click Edit.
9. Clear the attribute, and then use the following filter to configure the attribute:
   (&(objectCategory=person)(objectClass=user)(mailnickname=*)(homeMdb=*))
10. Click OK .

Solution was been created with the help of the following article:http://technet.microsoft.com/en-us/library/aa998426.aspx

note:You must enter the attribute without any spaces, exactly as it is.

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