A mail gateway is a security system that sits between the internet and an organization’s users to inspect, filter, and route email. It may block spam, scan for malware, enforce attachment and link policies, and verify sender identity using controls such as SPF, DKIM, and DMARC. Because it handles inbound and sometimes outbound mail, it is part of the trust boundary for email.
Mail gateways matter because email remains one of the most common attack paths for phishing, malware delivery, credential theft, and business email compromise. Defenders rely on gateways to stop malicious messages before they reach inboxes, reduce exposure to dangerous content, and log suspicious activity for investigation. Attackers, in turn, try to bypass these controls with spoofed domains, compromised accounts, encrypted attachments, or payloads designed to evade scanning. If a mail gateway has a flaw, the impact can be broad: a single weakness may expose many users at once or weaken the organization’s first line of defense.