The test email works to certain addresses but not others. Why?

If configured to send email directly, this likely means NG Firewall can only reach some email servers. Usually this is due to ISP blocking port 25. If configured to send email with an SMTP relay, this likely means that your SMTP relay is not configured to allow NG Firewall to relay to all addresses. The SMTP relay must allow NG Firewall to relay email to all addresses.