CakePHP 2 - CakeEmail代理设置

Question

I am using CakePHP2 and the default Email Config is as below

public $smtp = array(
        'transport'=> 'Smtp',
        'from'      => array('emailaddress'=> 'displayname'),
        'host'     => 'serveraddress',
        'port'     => 25,
        'timeout'  => 30,
        'username' => 'username',
        'password' => 'password',
        'client'   => null,
        'log'      => TRUE,
        'delivery'=> 'smtp'
    );

However, i would wish to define a proxy server for sending this e-mails.

Can anyone help me defining Proxy Server for sending E-Mails or can i configure a proxy server for all Cake Requests?