Silverpop Rate Limits

Acoustic Campaign (the platform Silverpop Engage became) does not throttle by request rate. It throttles by CONCURRENCY, and it publishes no rate-limit response headers at all. The documented ceiling is 10 concurrent authenticated requests per ORGANIZATION — not per key, per user, per list or per endpoint — and the 11th in-flight request is rejected outright rather than queued and retried. Callers on the legacy JSESSIONID session auth are additionally capped at 20 active login sessions per organization, and the docs tell you to call the Logout API to release one, because sessions abandoned without a logout can block new session creation for several minutes. An agent integrating this API therefore cannot read a budget off a response header; it has to cap its own in-flight fan-out at 10 and treat rejection as the signal.