Unable to send outbound email to a particular domain.
From message audit log, it is showing this error: "530 must issue a starttls command first"
Check and ensure SMG delivery setting is configured to attempt TLS encryption to deliver message.
1. Go to Administration > Configuration > Host > SMTP > Advanced Settings > Delivery.
2. Tick "Attempt TLS encryption for delivery of all messages"
3. Click Continue and Save.