During the mailbox enabling process, the legacyExchangeDN attribute is not being stamped on the target objects although the target object already being mailbox enabled.
The target object also being stamped with source x500 address. Since the target objects do not have a legacyExchangeDN stamped on them, as result target x500 address is not being stamped on the source objects as well.
This is due to the "mail" attribute on the source objects were not populated. In some environment the AD objects were created with a script which did not populate the mail attribute on it., as the result.
Populate the "mail" attribute information for affected source objects. Subsequent Directory Synchronization, the legacyExchangeDN attribute in target object will be stamped.
AD tools, such as ADModify, may be used to modify large numberof objects.