WildDuck

WildDuck is a modern IMAP/POP3 mail server designed for large-scale deployments. It stores all mail data in a sharded and replicated MongoDB backend and runs stateless Node.js instances that can be scaled behind a load balancer.

Key Features

• First-class IMAP and POP3 support for standard mail clients
• MongoDB-backed storage for messages and metadata, designed for sharding and replication
• Stateless architecture for horizontal scaling and high throughput
• REST API to manage accounts, access, filtering features, and server settings
• Unicode-first support for internationalized email addresses and folders
• Built-in account security features such as application-specific passwords, rate limiting, and MFA helpers (TOTP/U2F)
• Optional support for storing a user GPG public key to encrypt stored emails

Use Cases

• Hosting IMAP mailboxes for large organizations with many users and large quotas
• Building an API-driven mail platform where provisioning and operations are automated
• Running a scalable mail storage and access layer alongside separate SMTP components

Limitations and Considerations

• Opinionated design may not fit setups that depend on traditional MTA/IMAP stacks and features like Sieve-based workflows
• Typically used together with additional components (for example, an SMTP server) to form a complete mail system

WildDuck is a strong fit when you need an API-controlled, horizontally scalable IMAP server architecture rather than a classic single-host mail stack. For high-user-count installations, its stateless design and database-backed storage simplify scaling and operations.