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Stalwart

LicenseFOSS problemsMaintenanceSSOUsersTested
AGPL-3.0 (core) / Stalwart Enterprise License (add-ons)Multi-tenancy, the fuller admin dashboard, and AI-assisted features are Enterprise-License-only, not AGPLActive✅Unlimited (mailbox-count limited under an Enterprise license)❌

Description​

Stalwart is an all-in-one mail and collaboration server written in Rust — SMTP, IMAP, JMAP, POP3, and CalDAV/CardDAV/WebDAV in a single binary, rather than mailcow's approach of wiring together several existing projects.

Features​

  • Single binary, one codebase — SMTP, IMAP, JMAP, POP3, and CalDAV/CardDAV/WebDAV all live in one Rust codebase instead of a stack of separate daemons.
  • JMAP-first design — built around the newer JMAP protocol from the outset, with IMAP/POP3/SMTP supported alongside it rather than being the sole focus.
  • OpenID Connect both ways — can act as an OIDC provider itself, or delegate authentication to an existing identity provider; mail clients then authenticate via OAUTHBEARER using a token obtained from that provider.

Usage patterns​

  • Full mail server for a domain, on modern protocols — the same core use case as mailcow (receive, send, filter), but as a single Rust service instead of a multi-container stack.
  • Open-core: some features are Enterprise-only — multi-tenancy (multiple organizations/domains on one instance), the fuller admin dashboard, and AI-assisted email processing sit behind the Stalwart Enterprise License, not the AGPL core.

License and governance​

Dual-licensed: AGPL-3.0 for the Community Edition, plus the proprietary Stalwart Enterprise License (issued per domain, with a mailbox-count limit) for multi-tenancy, admin dashboards, and AI-powered features. The mild end of open-core — closer to Coolify or Pulumi's shape than a bait-and-switch pattern: the core mail engine (SMTP/IMAP/JMAP, single-domain, single-tenant) is never gated, only the productised multi-org layer on top. Single-vendor governed by Stalwart Labs, not a foundation.

Conclusion​

Why you would — a single, modern-protocol mail server (JMAP alongside IMAP/SMTP) without mailcow's multi-container footprint, genuinely usable and modifiable under AGPL for single-domain use.

Why you wouldn't — the Enterprise License gate on multi-tenancy means running mail for more than one organization/domain on one instance pushes into paid territory; mailcow doesn't draw that line.