A: Microsoft Outlook has long allowed users to arrange email by conversation. Outlook 2010, however, improves on that presentation, as you can see in the tip "Q: How can I have Microsoft Outlook 2010 ignore future messages in an email thread?"
As users reply to conversations, original content from the thread gets included in responses. The result, especially in a long conversation, is a lot of repeated content across all the messages. Outlook 2010 adds a new feature called Conversation Clean Up. This feature works for any type of account in Outlook 2010; however, if you're using Microsoft Exchange Server, whichever version, Outlook 2010 must be in cached mode.
Conversation Clean Up scans the thread for messages that are wholly contained within other messages, then deletes the older one. To me, this sounds scary. I'm trusting Outlook to remove content based on its assessment that the content is redundant. Microsoft included some options to limit what gets removed. These options are found in the Mail section of the Outlook Options window, which is found by navigating to File, Options. Figure 1 shows the different options for the Clean Up process. Outlook by default prevents Clean Up from removing categorized or flagged messages, messages altered by a reply, and digitally signed messages. You can also opt to exempt messages that haven't been read.
To Clean Up a conversation, right-click a conversation within any mail folder and select Clean Up Conversation from the context menu. You'll then see a confirmation dialog box, as Figure 2 shows.
Figure 2: Conversation Clean Up confirmation message
Running Clean Up moves redundant messages to the Deleted Items folder unless you've defined an alternate location, which you can do in the Mail section of Outlook Options, as seen in Figure 1. If you don't see the Clean Up option, it's likely because you aren't right-clicking at the top of a conversation. The Clean Up option isn't visible when you right-click a message within the conversation view.
You'll see the full power of this feature at work when you select Clean Up from the Home tab on the Ribbon. From here, you can apply the Clean Up to a conversation, a folder, or a folder and its subfolders. If this option has never been used, or even not recently used, this process can significantly reduce the size of a mailbox. If you're concerned about what's being removed, you can certainly review the Deleted Items folder or whatever custom destination folder you've assigned.
As the Conversation Clean Up feature deletes messages in a thread, it also removes some of the ability to troubleshoot based on those messages on the client side, including working with header information. The body and envelope of a message deleted by the Clean Up process still reside in an existing message; however, the SMTP header information is no longer available. The trade-off is that Conversation Clean Up can help reduce the volume of messages in your mail folders.