Why concurrency is important for large senders

Photo by Giuseppe Milo

Senders of large email volumes rely on the concept of multiple virtual queues for efficient delivery; separating email from different customers, to different destinations. This guarantees that for example one customer with a gigantic queue, or a destination which is tarpitting, doesn’t affect delivery of other traffic. Email transactions can take some time to complete, often as a result of content filtering at the destination. It is therefore necessary to support many simultaneous (concurrent) connections in each virtual queue, to support the throughput required for timely delivery. If each message takes an average of 1 second to deliver, 50 concurrent connections are needed to deliver 50 email per second.

The total maximum concurrency needed is the number of virtual queues multiplied with the desired concurrency for each queue. Traditional MTAs uses a process- or thread-based connection model. Those models scales poorly, and consequentially struggles to support more than a few thousand concurrent connections per server. This limitation becomes a problem as the system is scaled up, leading to bottlenecks or low customer density per MTA instance.

The Halon MTA uses a modern event-based connection model, sometimes called asynchronous or non-blocking. It scales exceptionally well; supporting tens of thousands of concurrent connections. Multiple worker threads are used to leverage all available CPU cores. This eliminates bottlenecks and can reduce the number of instances needed, which translates to lowered costs and simplified management.

Being asynchronous have benefits for receiving and processing inbound traffic as well. Consider the scenario where thousands of slow, or even idle, clients are connected to an MTA. Traditional MTAs would have to defer new connections from potentially legitimate senders, disturbing the service. Asynchronous MTAs like Halon are on the other hand be able to cope with a significantly large number of concurrent connections, and would handle the scenario above without breaking a sweat.

If you’re interested in learning more, don’t hesitate to reach out.

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