Webform response: "Illegal information. Data not submitted."
It seems that message is triggered by the _webform_submission_spam_check function. Looking at the code, it seems to check the following regular expression:

'/(b?cc\s?:)|(content\-type:)/i', $to . "\n" . $subject . "\n" . $from . "\n" . $headers


Maybe some of the submissions contain "cc" or "bcc" or "content-type"?