Codeigniter Form Validation Callback Function

I'm trying to create a form validation callback function but I'm having a little trouble getting my head around it.

What I am trying to do is create a contact form where with a join the mailing list option. If the option to join the mailing list is checked I want the name and email of the person to be added to the mailing list database. This part works perfectly however I also want the function to check the database to ensure that the email address being added is unique and this is the bit that I just can't get my head around.

Controller:

public function contact()
{
    $this->load->helper('form');
    $this->load->library('form_validation');

    $this->form_validation->set_rules('name', 'your name', 'required', array('required'=>"<p class='required'>Please provide %s</p><br>"));
    $this->form_validation->set_rules('email', 'your email address', 'required', array('required'=>"<p class='required'>Please provide %s</p><br>"));

    if($this->form_validation->run() == FALSE)
    {
        $this->load->view('templates/headder');
        $this->load->view('contact');
        $this->load->view('templates/footer');
    }
    else
    {
        $this->load->library('email');

        $name = $this->input->post('name');
        $email = $this->input->post('email');
        $phone = $this->input->post('phone');
        $message = $this->input->post('message');
        $list = $this->input->post('mailing_list');

        $email_message = "Name: $name<br>Email: $email<br>Phone: $phone<br>Message:<br>$message";

        $this->email->initialize();
        $this->email->from($email, $name);
        $this->email->to('myaddress@mydomain.co.uk');
        $this->email->subject('New Query');
        $this->email->message($email_message);
        $this->email->send();

        if($this->email->send()){
        $this->load->view('send_error');
        }
        else
        {
            if($list == 'no')
            {
            $this->load->view('sent');
            }
            else
            {
                $this->form_validation->set_rules('email', 'Email', 'is_unique[mail_list, email]');


                if($this->form_validation->run() == FALSE)
                {

                $this->load->model('mailing_listm');
                $this->mailing_listm->add_name();
                $this->load->view('sent'); 
                }
                else
                {
                $this->load->view('contact');
                }

            }
        }
    }
}

Error Message:

A Database Error Occurred

Error Number: 1064

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' email 'myaddress@mydomain.co.uk' LIMIT 1' at line 3

SELECT * FROM `mail_list`, `email` WHERE mail_list, email 'myaddress@mydomain.co.uk' LIMIT 1

Filename: libraries/Form_validation.php

Line Number: 1134

Hopefully someone will be able to let me know what daft thing I've done this time.

Also, This function is turning into a bit of a monster, it's the most complicated thing I've every tried to write. Is there any way that I can split it out so that it is made up of several smaller functions instead of one gargantuan one?

Thanks,

EDIT I have updated my code in line with the comment below about using is_unique however now I am receiving an error message.