Explanation
Splunk Enterprise Security knows the local customer domain names so it can detect internal vs. external emails by using the Corporate Web and Email Domain Lookups. These are lookup files that contain the list of domains that are considered internal or corporate for the organization. The Corporate Web and Email Domain Lookups are edited during the initial configuration of Splunk Enterprise Security, and they are used to enrich events with the tag=internal_web or tag=internal_email fields. These fields indicate whether the web or email activity is internal or external, and they are used by dashboards and correlation searches in Splunk Enterprise Security to monitor and analyze the web and email traffic. References = Corporate Web and Email Domain Lookups Configure web and email domains in Splunk Enterprise Security Detecting Typosquatting, Phishing, and Corporate Espionage ... - Splunk