Email Journaling is the ability to record all email communication in an organization.
In Exchange 2003, there are three different type of journaling:
Envelope Journaling is the most suitable as it records data about all recipients that a message is delivered to. It allows you to archive envelope headers that include information about recipients, BCC recipients and members from distribution groups. Envelope Journaling is the only type journaling available in Exchange 2007.
In Exchange 2003 Journaling was implemented on individual mailbox stores on each physical server.
If you wanted to Journal message for a single recipient, either you had to journal all the recipients on that mailbox store or you had to create a new mailbox store for that specific user. In case we wanted to journal all recipients in the organization, we had to configure journaling on each mailbox store.
Journaling is flexible in Exchange Server 2007. Journaling can be triggered per database, per distribution list, or per user. Microsoft Exchange Server 2007 lets you record all e-mail messages that pass through computers that have the Hub Transport server role installed.
In Exchange 2007 all Hub Transport servers contain a transport agent called the journaling agent. You can configure it to journal e-mail messages that are sent or received by individual users or by various departments within or outside the exchange organization.
If all the messages that pass through the Hub transport, match the defined criteria that was configured in the journal rule, a journal report will be created and delivered to the Journaling Mailbox
Journaled messages are wrapped in journaling envelopes that contain detailed information about who the message was addressed to, who the message was from, the subject of the message, and other useful information.
Once the Categorization process is complete, the Journaling agent will act on messages, it allows the agent to determine if the message was sent directly to a recipient or if the message was received via distribution group expansion. It can also tell whether the recipient was on the To, the Cc, or the Bcc lines of a message that originated from within the Exchange Server 2007 organization.
The journaling mailbox can be a valid Simple Mail Transfer Protocol (SMTP) address. By specifying an SMTP address, you can store journaled messages in Microsoft Exchange, in a Microsoft Windows SharePoint Services site, or in other message archival solutions
Exchange 2007 Provides two Journaling options:
Standard journaling - Standard journaling enables the Journaling agent in Exchange 2007 to journal all messages sent to and from recipients and senders that are located on a specific mailbox database on a computer running the Mailbox server role. Standard journaling is also called per-mailbox database journaling.
Premium journaling - Premium journaling enables the Journaling agent in Exchange 2007 to use rules that you can configure to match the specific needs of your organization. You can create journal rules for a single mailbox recipient or for entire groups within your organization. Premium journaling is also called per-recipient journaling.
You must have an Exchange Enterprise Client Access License (CAL) to use premium journaling
Both standard and premium journaling use the Journaling agent located on Hub Transport servers. When you enable standard journaling on a mailbox store, this information is saved in the Active Directory directory service and is read by the Journaling agent. Journal rules configured with premium journaling are saved in a similar manner
Exchange 2007 supports journaling in a mixed Exchange 2007 and Exchange 2003 organization. Exchange 2007 can read the Exchange 2003 journaling configuration that is present on Exchange 2003 mailbox databases and journal messages to either an Exchange 2003 or Exchange 2007 journaling mailbox. Exchange 2003 can't read the journaling configuration that is used by Exchange 2007. However, Exchange 2007 stamps journaled messages and journal reports with properties that Exchange 2003 can read and understand. If a message has already been journaled by Exchange 2007 and the journal reports are sent to the same journaling mailbox, Exchange 2003 doesn't journal the message again. If a message is a journal report, Exchange 2003 treats the Exchange 2007 journal report as if it was an Exchange 2003 journal report.
There is an interesting article by David Strome on Exchange 2007 Journaling: