X_OUTBOUND_THROTTLED
Means
an outbound HTTP call was told to wait
Typical cause
the vendor answered a retryable status (429, 503) with a Retry-After header, and outbound()'s attempts or deadline ran out before the stated delay. meta.retryAfterSeconds carries the delay
Fix
wait meta.retryAfterSeconds before calling meta.vendor again, or lower the call rate at the site that calls outbound()