$headers .= "Importance: Highrn";
$headers .= "MIME-Version: 1.0rn";
$headers .= "Content-type: text/html; charset=iso-8859-1rn";
$headers .= "Content-Transfer-Encoding: 8bit";

IF(mail($email, $subject, $message, $headers, " -f".$email) === TRUE)...

...MIME-Version: 1.0rncontent-type: text/html; charset=iso-8859-1... - FAIL

...MIME-Version: 1.0rn content-type: text/html; charset=iso-8859-1... - TRUE

IF(mail($email, $subject, $message, $headers))...