Symptom: delivered events lag sends by hours; the webhook shows deferred events with attempt counts climbing.
Cause candidates:
1. Recipient-server throttling (421/450). Confirm: deferral responses naming policy or frequency, concentrated on one or two domains.
2. New or re-warmed IP under evaluation. Confirm: deferrals spread across domains on a fresh dedicated IP.
3. Reputation-driven throttling. Confirm: deferrals plus elevated spamreports and weak engagement.
Fix:
1. Do not resend. Deferred messages retry automatically for up to 72 hours; duplicates from manual resends annoy recipients and worsen throttling.
2. Reduce sending rate to the deferring domains.
3. On a new IP, follow the warmup curve instead of forcing volume.
4. For reputation throttling, fix engagement and list hygiene; the throttle is a symptom.
Verification: attempt counts stop climbing and deferred events convert to delivered within the retry window.