If that's not the reading style you prefer for your email conversations, you can set up Mac OS X Mail not to start with the latest message on top but with the first in the thread.
Make Mac OS X Mail Show Conversations Chronologically from Top to Bottom
To set up Mac OS X Mail so that the most recent message in a conversation appears at the bottom (and the oldest on top):
- Select Mail | Preferences… from the menu in Mac OS X Mail.
- Go to the Viewing category.
- Make sure Show most recent message at the top is not checked.
- To see conversations in reverse chronological order, have Show most recent messages at the top checked.
- Close the Viewing preferences window.
(Updated July 2011)