How do compromised ESP accounts cause widespread spam?

ESP accounts carry established reputation. Compromised accounts send spam through the legitimate sender's authentication and IP reputation, initially bypassing filters that would block unknown senders.

Scale amplification: ESPs enable high-volume sending. A single compromised account can send millions of spam messages before detection. The account owner's contacts and lists may also be targeted.

Collateral damage: spam from compromised accounts damages the legitimate sender's reputation, potentially affecting their deliverability permanently. ESP may suspend the account, disrupting legitimate business communications.