Kurrier is a self-hosted unified communication platform designed to aggregate email, calendars, contacts, and cloud storage into a single workspace. It functions as a multi-tenant SaaS application, supporting various protocols including IMAP/SMTP for email, CalDAV/CardDAV for scheduling and contacts, and WebDAV/S3 for file storage. The system emphasizes data ownership and privacy, allowing users to manage multiple email identities and storage volumes within isolated workspaces. The architecture follows a monorepo structure with a clear separation of concerns. The frontend (`apps/web`) is built on Next.js 16 with React 19, utilizing Server Actions for business logic and a Nitro-based worker (`apps/worker`) for background jobs like IMAP sync and email delivery. Data persistence is handled by PostgreSQL with Drizzle ORM, while Redis manages job queues and caching. The platform integrates with external providers via a strategy pattern, supporting services like Google Workspace, AWS SES, and various SMTP providers. Key components include a robust provider abstraction layer for pluggable email and storage backends, a centralized encryption vault for sensitive credentials, and a standardized Server Action wrapper for consistent error handling. The codebase demonstrates high craftsmanship in its sync logic and multi-tenancy implementation, though it carries some technical debt related to type safety and complex DAV/IMAP state management.