[eluser]CroNiX[/eluser]
Show your form element that this rule is being applied on. Something is strange with your mail test. It says "invalid string" where it should be showing the value of the form field.
Also remove the last closing php tag - they are unnecessary and can cause problems if whitespace appears after them - so it's best to just not use them.
You'll notice none of CI's libraries or anything else have a close php tag.
