What is connection throttling, and how can you detect it?

Connection throttling limits access rate:

What it means:

Server limits concurrent connections per sender. Limits connections per time period. Excess attempts rejected or delayed.

Detection signals:

Connection refused: Too many connections. 421 responses: "Too many connections." Increasing delays: Connections slower to establish. Queue buildup: Messages waiting for connections.

Metrics to monitor:

Connection success rate. Time to establish connection. Concurrent connections in use. Queue depth over time.

Response strategies:

Reduce concurrent connections. Implement connection pooling. Spread sends over time. Improve reputation for higher limits.

Throttling is bandwidth control for SMTP. Detect it through connection patterns and adjust accordingly.