生产环境 MAIL_PROVIDER=stalwart，域是 agent.voru.org。本地 stub 只在 inbox 之间投递。Stalwart 入站是 POST /v1/inbound/stalwart，头 X-Internal-Key。
MAIL_PROVIDER=stalwart STALWART_URL=http://127.0.0.1:8081 STALWART_USER=admin STALWART_PASSWORD=... STALWART_SMTP_HOST=127.0.0.1 STALWART_SMTP_PORT=465 STALWART_PUBLIC_HOST=agent.voru.org # inbound: POST /v1/inbound/stalwart header X-Internal-Key # local only: MAIL_PROVIDER=stub # inbox-to-inbox + POST /v1/inbound/hook