Cause
There is already an active bulk redelivery for this destination.
What to do
Wait for it to finish.
application/problem+json
{
"type": "https://masnet.com/developers/errors/redelivery_active",
"title": "Redelivery active",
"status": 409,
"detail": "...",
"code": "redelivery_active",
"correlation_id": "..."
}