Secure Email Gateways (SEG) inspect and filter inbound and outbound email, blocking phishing, malware, spam, and business email compromise before delivery, and applying outbound controls like encryption and content policy. This market evaluates the full modern spectrum from the platform's perspective: classic MX-record gateways (Proofpoint, Mimecast, Barracuda, Cisco, Fortinet), API-based integrated cloud email security challengers (Abnormal Security, Sublime Security, StrongestLayer, Inception Cyber), suite-native protection (Microsoft Defender), and the human-layer vendors (KnowBe4, Hoxhunt) buyers weigh alongside the gateway.