wildduck

WildDuck is a modern mail server that implements IMAP and POP3 on top of Node.js and a sharded, replicated MongoDB backend. All data, including messages and user metadata, is stored in MongoDB, allowing the service to scale horizontally by adding stateless instances behind a load balancer. The design follows Gmail’s feature set, offering full IMAP4rev1 support with extensions such as IDLE, CONDSTORE, COMPRESS, SPECIAL‑USE, UTF‑8 and quota handling, so standard email clients can connect without special configuration.

The server is managed entirely through a REST API, which handles account creation, mailbox configuration, message filtering, auto‑replies, DKIM signing and other administrative tasks without requiring manual edits of configuration files. Security is emphasized: the process runs without root privileges, has no direct filesystem access, and includes two‑factor authentication, WebAuthn, application passwords, PGP encryption and audit logging.

Because each component can be replicated—MongoDB replica sets, Redis Sentinel, and multiple stateless application servers—WildDuck avoids any single point of failure. It is distributed under the European Union Public License 1.2, is self‑hostable, free‑tier and open‑source, and is intended for deployments with a thousand or more mail accounts that need reliable horizontal scaling.