How do ESPs scale to send billions of emails?

Sending billions of emails requires massive distributed infrastructure. Here's how large ESPs scale.

Horizontal scaling:

Hundreds or thousands of MTA servers working in parallel

Traffic distributed across servers based on capacity and load

Adding servers increases total throughput linearly

IP pool management:

Thousands of IP addresses across multiple ranges

Intelligent assignment based on sender reputation and destination requirements

Automatic rotation and warmup for new IPs

Geographic distribution:

Data centers on multiple continents

Reduced latency by sending from locations closer to recipients

Redundancy and failover between regions

Queue architecture:

Distributed message queues handling millions of messages

Separate queues per destination domain for targeted throttling

Persistent storage to survive server failures

Intelligent routing:

Real-time reputation monitoring per IP/domain

Dynamic throttling based on destination feedback

Automatic rerouting around blocklisted IPs

Database infrastructure:

Sharded databases for subscriber data and tracking

Caching layers for frequently accessed data

Asynchronous processing for non-critical operations

It's a coordinated global shipping operation. Thousands of vessels, countless ports, all orchestrated by sophisticated logistics systems.