A: You can ignore future messages in an email thread in Outlook 2010 through the Ignore Conversation feature. When Outlook 2010 was in beta, the default view for the Inbox was changed from what it had been in previous versions to a new Conversation view. This format was created in response to customer requests for better email triage options. By the time Outlook 2010 was released to manufacturing (RTM), the classic view organized by date was set as the default, possibly the result of feedback during the beta. The new Conversation view did get some exposure during the beta, and many prefer that setting.
To change to the Conversation view in Outlook 2010, select View on the Ribbon, then select the Show as Conversations check box in the Conversation panel. You can also select Arrange By atop the mail folder column in the main UI for Outlook. The Show As Conversations option in the drop-down menu is available only when the folder is arranged by Date. When you change the view to or from Conversation view, Outlook prompts you to choose between All Folders or This Folder. If you select All Folders, the view change you are making will be applied to every email folder in your mailbox instead of just the one you're currently in.
There's a Group by Conversation option in previous versions of Outlook, but Outlook 2010 improves this view by presenting conversations in a different manner. For instance, it links a current message to the one within the conversation to which it is replying. If multiple messages within a thread represent replies to the same message, the conversation is referred to as a split conversation. Figure 1 shows the links to what message was replied to in a conversation. The orange line links the selected message to the replied message. The extraneous orange dot represents a message that replied to the same message as another, splitting the timeline flow of the conversation.
Figure 1: A split conversation shown in the Outlook 2010 interface
But what if an email conversation is no longer of interest to you? What can you do to rid your Inbox of that content? As Figure 2 shows, if you right-click a conversation while in Conversation view, the context menu shows an option labeled Ignore. Selecting this option removes the conversation from the folder and sends future messages belonging to this conversation directly to the Deleted Items folder. Outlook advises you, through a pop-up confirmation window, that this selection impacts all messages in the conversation. This feature really is misnamed—it should be called Delete Conversation because that's exactly what this option does when selected. The conversation gets moved to the Deleted Items folder, and future matching messages go directly to Deleted Items folder as well.
Figure 2: Selecting the Ignore option to ignore a conversation
The metadata describing the original folder location of a conversation doesn't remain with the conversation when it's deleted. If you choose to Ignore a conversation in error and want to correct this action, it's best reversed immediately with the Undo option. Otherwise, to return the conversation, you'll need to find it in the Deleted Items folder. From there, right-click the conversation and select Ignore again, which moves the conversation to the Inbox (not the original folder if it wasn’t in the Inbox). From there, you'll have to manually move it back to its original folder if it wasn't ignored from the Inbox.
Outlook 2010 uses a MailItem property called ConversationID to differentiate messages in Conversation view. If no ConversationID is present, Outlook resorts to the ConversationTopic property, which is the text of the Subject line. Outlook uses ConversationTopic as the ConversationID when the message originates from a client other than Outlook 2010 or did not pass through an Exchange 2010 server. When using ConversationTopic, should a user "highjack" a thread by changing the subject text in any way when replying or forwarding a message, a new conversation is rendered as far as the Conversation view is concerned.
MSDN has more information about working with the ConversationID property if you're developing applications that could benefit from it or if you want a more detailed overview of what happens behind the scenes: