Sengupta, David. Best Practice: Always Use Mailbox as Target for Exchange Journaling for Compliance Purposes. July 23, 2004.

Have been doing some reading lately about message jouraling and archiving for compliance purposes, specifically to do with Microsoft Exchange environments.  First of all, some definitions. 
  • Journaling - refers to the ability to record all communications.
  • Archiving - refers to the ability to remove content from native data storage (i.e. Exchange databases) and store it elsewhere (i.e. File system, SQL, Oracle, mySQL, Tape, HSM, nearline/offline storage, etc.) to reduce capacity.

If you're using the built in Exchange journaling functionality, it's always a best practice to specify a mailbox as a target journaling container.  In other words, don't use a Public Folder (PF) or Contact/Custom Recipient as the target for journaling.  This is because cetrtain types of messages don't survive transport to a Public Folder (by design) including NDRs, etc.  It's always a best practice to journal to a mailbox, and if you need to, to use a rule set on the journaling mailbox to forward/redirect journaled messages to another location (such as an archive).

As an aside, note that there is some blurring of nomenclature when in comes to journaling in Exchange 2000 and Exchange 2003.  The settings to enable message journaling (at the store level only) allow you to "archive all messages sent or received by mailboxes on this store" to a specified recipient.  If you think about what's actually happening here, this is simply journaling and not archival.  But I digress ...

To summarize, always use a target mailbox if you're enabling message journaling in Exchange 5.5, Exchange 2000 or Exchange 2003.  I'll probably flip this into a Microsoft Community Solutions KB Article when I get a chance.