This problem can occur if the Exchange 2000 ADC Local.map file has been updated to include one or both of the following mappings: "Mailbox to user" or "Custom Recipient to Contact":
local#remote-address$person$top#contact$organizationalperson$person$top#cn#description###0#
local#remote-address$person$top#contact$organizationalperson$person$top#cn#displayName###0#.
In addition, the Exchange Server 5.5 recipient object has a comma (,), equal sign (=), or plus (+) in the directory name (on the
Advanced tab in the Exchange Server 5.5 mailbox).
Those objects that have a comma (,), equal sign (=), or plus (+) in the directory name (LastName, FirstName) in Exchange Server 5.5 are replicated to the ADC as mail-disabled in the Active Directory. The
legacyExchangeDN attribute on the Active Directory object reads
ADCMailDisabledbyADC. The same problem applies to "Custom Recipient to contact" mappings in the Local.map file.