Contact Form 7: validate email field with more than one address

I would like to be able to send emails from a CF7 form to more than one recipient using an input field in the form to obtain the email addresses. The problem is that the email field of CF7 only validates a single email address not two separated by comma, which are considered invalid.

I COULD use a text field, but then there is no validation at all of correct email syntax. How could I add a field type "emails" with validation of single or multiple emails?

The easiest way to achieve this is to use a std text field and create your own validation. You need to hook the cf7 plugin's validation filter for text fields,

add_filter( 'wpcf7_validate_text*', 'my_vaidated_list_of_emails', 10, 2 );
add_filter( 'wpcf7_validate_text', 'my_vaidated_list_of_emails', 10, 2 );
function my_vaidated_list_of_emails( $results, $field_tag ) {
    // check this is your field
    if ( 'email_lists' === $field_tag->name && isset( $_POST['email_lists'] ) ) {
        // split the lists into an array of emails
        $emails = explode(  ',', $_POST['email_lists'] );
        $count       = 0;
        foreach ( $emails as $e ) {
            $count++;
            if ( false === filter_var( strtolower( trim($e) ), FILTER_VALIDATE_EMAIL ) ) {
                $results->invalidate( $tag, "Email no.{$count} in your list is invalid." );
                break; // from the loop, no need to go any further.
            }
        }
    }
    return $results;
}

Note: I haven't tested this, but this should do it.

Edit: changed the code to use the far simpler filter_var as suggested by andrew in the comments below.