An email service provider (ESP) is a platform that sends bulk, transactional, or automated messages for an organization, such as newsletters, alerts, password resets, and verification emails. It usually manages templates, recipient lists, delivery infrastructure, and tracking.
ESP security matters because users trust messages that come from familiar brands and reputable mail systems. If attackers compromise an ESP or its sending credentials, they can send convincing phishing emails from a legitimate-looking source, bypassing many suspicion cues and sometimes improving inbox placement through the platform’s sender reputation. Defenders protect ESP accounts with strong authentication, least-privilege access, domain authentication such as SPF, DKIM, and DMARC, and monitoring for unusual campaigns, template changes, or API use. In attacks, the ESP becomes a trust channel; in defense, it is a sensitive part of the communications supply chain.