Why by codeigniter email is going to spam folder with full information?

Hi i am using CodeIgniter with PHP technology and i am sending emails from application but emails are being sent in spam folder of recipients.

$this->load->library('email');

$config['protocol'] = 'sendmail';
$config['charset'] = 'utf-8';
$config['wordwrap'] = TRUE;
$config['mailtype'] = 'html';

$this->email->initialize($config);

$this->email->from('aaaa@domain.com', 'MYRegistration');
$this->email->to('abc@domain.com');

$this->email->subject('Congratulation ! You are a User.');
$this->email->message('html code');
$KsbMailStatus = $this->email->send();

This email contains some images and links also because this is a registration emails.