large attachment cannot be sent using codeigniter

Hi guys, 

I'm implementing a program of sending email using Codeigniter Email class and gmail smtp. I find it works when the attachment is small (e.g., less than 1MB) but if it's large (e.g., 5MB) then the attachment cannot be sent with the email, though the email itself can be sent without any errors. I checked my host (I'm using Ipage by the way), the attachment is successfully uploaded to the host, so it's not a problem of uploading. I also checked the php.ini on Ipage host, here are relevant parameters, they all seem to be OK:

memory_limit = 256M
max_execution_time = 120
upload_max_filesize = 8M
post_max_size = 20M

I tried using localhost it didn't work with large attachments either.

Could anybody tell me what the problem could be? Thank you very much!
There isn't anything special about the code, but i attach the code below just in case someone wants to see.

PHP Code:

$config['mailtype'] = 'html';
$config['charset'] = 'UTF-8';
$config['newline']    = "\r\n";//没有这句话就会出现send没响应的情况
$config['wordwrap'] = TRUE;
$config['protocol'] = 'smtp';
$config['smtp_host'] = 'ssl://smtp.googlemail.com';
$config['smtp_port'] = 465;
$config['smtp_timeout'] = 30;
$config['smtp_user'] = '******@gmail.com';
$config['smtp_pass'] = '*********';
                            
$this->load->library('email'); 
$this->email->initialize($config);                                                       
         
            
$this->email->from('****@gmail.com');                           
$this->email->to(*****);

$this->email->bcc('******');
$this->email->subject($order->subject);
$this->email->message($order->content);
if($order->attach)//if there is attachment
    $this->email->attach("path/to/attachement");

if($this->email->send()) 
{
}
else 
    $this->email->print_debugger();