SwiftMailer - Yii2无法发送电子邮件

'mailer' => [
    'class' => 'yii\swiftmailer\Mailer',
    'transport' => [
        'class' => 'Swift_SmtpTransport',
        'host' => 'smtp.example.com',
        'username' => 'your_username',
        'password' => 'your_password',
        'port' => '587',
        'encryption' => 'tls',
    ],
],

use Yii;

$mailer = Yii::$app->mailer;
$message = $mailer->compose()
    ->setFrom('from@example.com')
    ->setTo('to@example.com')
    ->setSubject('Hello')
    ->setTextBody('This is the plain text content of the email.')
    ->setHtmlBody('<b>This is the HTML content of the email.</b>');

if ($message->send()) {
    echo 'Email sent successfully.';
} else {
    echo 'Failed to send email.';
}