Symfony签名邮件报错：无法创建Email实例，如何解决？

报错信息 #

Unable to send message with the "Symfony\Component\Mailer\Transport\AbstractApiTransport" transport: Unable to create an Email from an instance of "Symfony\Component\Mime\Message" as the body is too complex.

问题背景 #

移除SMIME签名功能后邮件可正常发送，但为了让邮件服务商优先处理邮件，需要保留签名功能并解决该报错。

邮件配置 #

MAILER_DSN=roundrobin(mailgun+api://*****55643c7e*****f8361b5*****27c-e61ae8dd-7*****:sandboxef6*****2742745ce*****c048689*****.mailgun.org@default?region=us)

相关代码 #

use Twig\Environment;
use App\Core\MailStruct;
use Psr\Log\LoggerInterface;
use Symfony\Component\Mime\Email;
use Twig\Loader\FilesystemLoader;
use Symfony\Bridge\Twig\Mime\BodyRenderer;
use Symfony\Bridge\Twig\Mime\TemplatedEmail;
use Symfony\Component\Filesystem\Filesystem;
use Symfony\Component\Mailer\MailerInterface;
use Symfony\Component\Mime\Crypto\SMimeSigner;
use Symfony\Contracts\Service\Attribute\Required;

public function __construct(MailerInterface $mailer)
{
    $this->mailer = $mailer;
}

public function send(MailStruct $mail): void
{
    
    $email = (new TemplatedEmail())
    ->from('mailgun@*****.mailgun.org')
    ->to('b***@yahoo.com')
    ->replyTo('mailgun@*****.mailgun.org')
    ->priority(Email::PRIORITY_HIGH)
    ->subject('Time for Symfony Mailer!')
    ->htmlTemplate('test.html.twig')
    ->locale('de')
    ->context([
        'expiration_date' => '77',
        'username' => 'stellaGregory',
        'address' => '6 Kamasojo',
    ]);

    $loader = new FilesystemLoader('../templates/');
    $twigEnv = new Environment($loader);
    $twigBodyRenderer = new BodyRenderer($twigEnv);
    $twigBodyRenderer->render($email);

    try {
        $signer = new SMimeSigner(__DIR__.'/certificate/certificate.crt', __DIR__.'/certificate/private.key');
        $email = $signer->sign($email);
        $this->mailer->send($email);   
    } catch (\Throwable $th) {
        throw $th;
    }
}

解决方案 #

核心原因 #

报错根源在于你使用的Mailgun API传输（mailgun+api://）属于AbstractApiTransport，这类传输需要将邮件解析为结构化的Email对象来调用服务商API，但SMIME签名后，原TemplatedEmail会被转换为Symfony\Component\Mime\Message——这是一个完整的MIME消息对象，结构过于复杂，无法被API传输解析。

解决办法 #

切换到SMTP传输协议，SMTP传输直接发送完整的MIME消息，不需要解析邮件结构，完美支持签名后的复杂Message对象。

1. 修改MAILER_DSN配置
   将原API协议的DSN替换为SMTP协议的DSN，格式如下：

MAILER_DSN=mailgun+smtp://SMTP_USERNAME:SMTP_PASSWORD@default?region=us

• SMTP_USERNAME和SMTP_PASSWORD可在Mailgun控制台的「SMTP凭证」板块获取（通常用户名是postmaster@your-domain.mailgun.org，密码是随机生成的字符串）。
• 如果使用roundrobin负载均衡，格式为：

MAILER_DSN=roundrobin(mailgun+smtp://SMTP_USERNAME:SMTP_PASSWORD@default?region=us)

2. 代码优化（可选）
   原代码中手动实例化Twig环境和渲染器不符合Symfony最佳实践，可改为注入Twig服务：

// 构造函数注入Twig\Environment
public function __construct(MailerInterface $mailer, private Environment $twigEnv)
{
    $this->mailer = $mailer;
}

public function send(MailStruct $mail): void
{
    $email = (new TemplatedEmail())
        ->from('mailgun@*****.mailgun.org')
        ->to('b***@yahoo.com')
        ->replyTo('mailgun@*****.mailgun.org')
        ->priority(Email::PRIORITY_HIGH)
        ->subject('Time for Symfony Mailer!')
        ->htmlTemplate('test.html.twig')
        ->locale('de')
        ->context([
            'expiration_date' => '77',
            'username' => 'stellaGregory',
            'address' => '6 Kamasojo',
        ]);

    // 无需手动渲染，TemplatedEmail会自动通过Twig服务渲染
    try {
        $signer = new SMimeSigner(__DIR__.'/certificate/certificate.crt', __DIR__.'/certificate/private.key');
        $email = $signer->sign($email);
        $this->mailer->send($email);   
    } catch (\Throwable $th) {
        throw $th;
    }
}

内容的提问来源于stack exchange，提问作者DevOPlug