The information used to determine the email address associated to an EV archive is stored in the EV Vault Store database; the column used is the legacy Mailbox Distinguished Name (DN).  The DN data is inserted at the time the Archive is created and may not reflect the current Active Directory information.  If Users are migrated to new Exchange servers anytime after the creation of the EV archive, the DN data will not match between the EV Vault Store database and Active Directory(AD). 

eDiscovery uses the DN data stored in the EV Vault Store database to match the DN in Active Directory to extract the Email Address. When the legacy and current DN's do not match, the Email Address field will not populate.

This condition will happen when original mailbox distinguished name that was recorded by EV during the creation of the archive, differs with Active Directory.
This condition should only occur during the initial EV Archive discovery. Once resolved and EV discovery is scheduled, the issue will not reappear.

RESOLUTION:
As long as the user exists in Active Directory, the column can be populated.
Contact Customer Support for assistance to populate this column.