How do attackers gain access to sending infrastructure?

**ESP** account compromise: attackers steal credentials to **ESP** accounts, gaining ability to send from the victim's reputation and authenticated domains. Phishing **ESP** users yields high-value access.

**API** key theft: exposed or stolen **API** keys enable sending through victim infrastructure without full account access. Keys in code repositories, logs, or breached systems enable abuse.

**MTA**/server compromise: attackers gain server access through vulnerabilities or stolen credentials, then configure unauthorized sending. Internal network compromise can reach mail infrastructure.