A user had a complaint about categorizing email messages:
When I add a category to a message, every message in the thread thereafter will also be marked with the category. I use categories to set action items for specific messages and automatically propagating the categories to future messages completely defeats my purpose.
Whether or not every message in the thread is categorized depends on how you set the category. When you categorize the message’s conversation group header, all messages in the thread (including messages received later) will be categorized.
To avoid this, you need to categorize just the message. While this easy when you have multiple messages in the thread – expand the thread, select the message and categorize it – its more difficult when there is a single message in the thread as the message is the conversation group too. To avoid categorizing the entire thread, open the message then set the category, or turn off Show as Conversations (on the View ribbon) before setting the category.