Cause
There is already a test in progress for this destination.
What to do
Wait the seconds in Retry-After.
application/problem+json
{
"type": "https://masnet.com/developers/errors/test_in_progress",
"title": "Test in progress",
"status": 429,
"detail": "...",
"code": "test_in_progress",
"correlation_id": "..."
}