In Outlook 2010, you can insert comments into email message by using shortcut key. Please look at the following instructions.
Insert Comment Callouts Into Email Message In Outlook
1. Compose your new email by clicking New Email button.
Or open an email you want to reply, reply all or forward, and then click Reply, Reply All or Forward to edit.
2. Highlight (double click on or select) the word or text which you want to comment on.
3. Then press Ctrl + Alt + M to insert the comment. When the comment callout showing up, add your comment on it.
4. If you want to remove the comment, put the cursor on the comment and right click on it. Then select Delete Comment option to remove the comment.