I still can’t see where you went wrong in the From set-up as you have not provided much detail as needed.
The info provided looks complete to me. There is your-message in FORM section and [your-message] in MAIL section Message body.
it could be that the sender wrote “1” in the message field of your contact Form.
It could also be a JavaScript conflict with either the current WordPress theme or one of the other plugins they are using.
Step by step approach to problem solving
1. Switch temporarily to a WordPress default theme (Twenty Eleven etc.) and see if the problem goes away.
If it does, you’ve most likely got a problem with your current WordPress theme.
If switching to default theme doesn’t help it could be one of the plugins you are using that is causing the problem.
2. Disable all plugins other than Contact Form 7
If doing that solves the problem, you can then add back each item – one by one, until you find the cause of the problem.