If you have set up Messaging so that the original message is included in your replies, you can also flag the lines of the original message with the ">" (greater than) symbol. (This will visually distinguish the contents of the original message from the contents of your reply.)
From the Options and Styles... menu, select Preferences under My Options.
On the User Preferences page, locate Reply message (original message indicator) and select ">".
Click the Save button at the bottom of the table you are working in when finished.
Note that if you select ">", but have not selected Include original message under Replying, the original message will not be included or flagged.