How are you trying to view the archived mail?

This technote explains it a bit,

Article URL http://www.symantec.com/docs/TECH53025

For BCC viewing behavior
The BCC recipient information can only be displayed in the review set of searches of Compliance Accelerator (CA) or Discovery Accelerator (DA) 2007 Service Pack 1 or later or CA / DA 8.x with the ECM API enabled.  The following viewing behaviors are the current expected behaviors:
- Exchange 2003 message with DL in the BCC field and the Journal Task's DL expansion setting set to either active or disabled
--- with neither Envelope or BCC Journaling active, nothing will be displayed
--- with Envelope Journaling active, DL expansion is forced to be active regardless of the Journal Task settings, the DL members will be displayed, but not the DL name
--- with BCC Journaling active, the DL name will be displayed, but the DL members will not be displayed

- Exchange 2007 message with DL in the BCC field
--- with either Standard or Premium Journaling enabled, DL name will be displayed, DL members will not be displayed (the same as Exchange 2003 BCC Journaling).  This behavior is due to Exchange 2007 DL expansion providing only the SMTP e-mail addresses of the DL members and not providing the member aliases.