在配置文件中保留默认电子邮件设置。
在你的控制器动作中做类似这样的事情：

use Cake\Mailer\MailerAwareTrait;
use Cake\Mailer\TransportFactory;
// ....
public function index()
{
    $users = $this->Users->find();

    foreach ($users as $user) {
        TransportFactory::drop('gmail'); // If you wish to modify an existing configuration, you should drop it, change configuration and then re-add it.
        TransportFactory::setConfig('gmail', [
            'host' => 'ssl://smtp.gmail.com',
            'port' => 465,
            'username' => $user->mail_username,
            'password' => $user->mail_password,
            'className' => 'Smtp',
        ]);

        $this->getMailer('Users')->send('user', [$user]);
    }
}

或试试这个：

$this->getMailer('Users')
->drop('gmail')
->setConfig('gmail', [
    'host' => 'ssl://smtp.gmail.com',
    'port' => 465,
    'username' => $user->mail_username,
    'password' => $user->mail_password,
    'className' => 'Smtp',
 ])
->send('user', [$user]);

阅读更多信息https：book.cakephp.org/4/en/core-libraries/email.html#configuring-transports
**注意：**出于安全原因，请确保不要将纯文本密码保存到数据库中