Often you will want to send in additional information to a question you have already sent. The best way to do this is to reply to your original message. This has three advantages:
You include the original message text, so people will know what you are talking about. Do not forget to trim unnecessary text out, though.
The text in the subject line stays the same (you did remember to put one in, did you not?). Many mailers will sort messages by subject. This helps group messages together.
The message reference numbers in the header will refer to the previous message. Some mailers, such as mutt, can thread messages, showing the exact relationships between the messages.