fix(smtp): 修复下游投递失败后 SMTP DATA 仍返回成功，导致上游误判为已发送 - #356

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/