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.