Error message after form validation runs successful

Hi,

is it possible to set an error message AFTER form validation runs successful?
The reason is, i want to include mailchimp and mailchimp blocks email adresses. if so, i want to show an error message.

PHP Code:

if ($this->form_validation->run() === true) {
    // connect to mailchimp
    // if mailchimp throw an error, show an error message
}