How do ESPs distribute sending load?

Distributing sending load efficiently requires balancing multiple constraints: infrastructure capacity, destination rate limits, IP reputation, and customer prioritization.

Queue-based distribution:

Messages enter centralized or distributed queues

Worker processes pull from queues and handle sending

Queue depth and latency guide resource allocation

IP assignment:

Algorithms match senders to appropriate IP pools

High-reputation senders get premium IPs

New/risky senders use isolated or lower-tier IPs

Load balances across IPs to avoid overloading any single address

Destination-aware throttling:

Separate queues or rate limits per recipient domain

Gmail, Yahoo, Microsoft each have different acceptance rates

Systems slow down to specific domains showing deferrals

Customer prioritization:

Time-sensitive messages (transactional) may get priority

Enterprise customers may have dedicated resources

Fair sharing prevents any single customer from monopolizing capacity

Dynamic adjustment:

Real-time monitoring of delivery feedback

Automatic scaling up/down based on traffic patterns

Rapid response to blocks or reputation issues

It's air traffic control for email. Thousands of flights scheduled, limited runway capacity, weather changing constantly. Good systems keep everything moving safely.