basically when wheels throws an error, it will try to send out an error email containing the environment and framework information. In this case there seems to be no smtp server defined, so CFML itself is spitting up its skull.
The error handling can be reworked so that the $mail() call is wrap in a try/catch so the 500 page will just be shown.
I agree that the owner of the website here needs a <CFERROR> tag, which tells the server what to do instead of showing that big error message.