Ever wondered how Gmail, Yahoo, and Apple iCloud manage to host hundreds of millions of email accounts reliably? How do they store petabytes of messages, survive hardware failures without losing data, and keep spam at bay across billions of daily deliveries?

This talk explores how to design and operate a large-scale email system using Stalwart, an open-source mail server built from the ground up for distributed deployments. Using a 1,024-node cluster as a concrete example, we will examine the architectural patterns that make planet-scale email possible, and how similar approaches are used by providers such as Apple iCloud.

The session covers the full stack of distributed email challenges: storing and indexing messages across a cluster, running spam and phishing filtering at scale without becoming a bottleneck, managing distributed MTA queues for reliable delivery, and load balancing IMAP, JMAP, and SMTP traffic across nodes. We will also look at how Stalwart handles cluster coordination, orchestration, and autoscaling, how to reason about failure scenarios before they occur, and how to adapt a deployment to fluctuating load in dynamic environments.

Attendees will leave with a practical understanding of how modern distributed email systems are built and operated, and how to apply these principles using open-source technology.