Stalwart: Can Open Source do Gmail-Scale Email?

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 begins with an introduction to Stalwart, a secure and scalable open-source email and collaboration server written in Rust. Stalwart is designed for modern deployments and supports both next-generation protocols such as JMAP as well as legacy standards including IMAP, SMTP, WebDAV, CalDAV, and CardDAV. We will also discuss the limitations of traditional mail software such as Dovecot and Postfix, and how Stalwart was built as a modern, unified replacement for both.

Using a 1,024-node cluster as a concrete example, the second part of the session explores how to design and operate a large-scale distributed email system. Topics include storing and indexing messages across a cluster, running spam and phishing filtering at scale, managing distributed MTA queues for reliable delivery, and load-balancing IMAP, JMAP, and SMTP traffic. We will also look at cluster coordination, orchestration, autoscaling, and how to reason about failure scenarios before they occur.

Attendees will leave with a practical understanding of how modern, planet-scale email systems are built and operated, and how these principles can be applied using open-source technology.