Kurrier is a self-hosted, multi-tenant communications workspace that combines email, calendar, contacts, and file storage into one product. It runs as a pnpm monorepo with three deployable apps: a Next.js App Router frontend (apps/web) using server actions and locale-prefixed dashboard routes for mail, calendar, contacts, drive, and platform settings; a Nitro-based worker (apps/worker) that exposes REST management APIs under /api/kurrier, inbound email webhooks for SES/Mailgun/Sendgrid/Postmark/Mailtrap, and CalDAV/CardDAV/WebDAV endpoints while consuming BullMQ/Redis background jobs for IMAP, Gmail, JMAP, and DAV sync, rule processing, search indexing, and scheduled sends; and a Fumadocs documentation site (apps/docs). Persistence is PostgreSQL via Drizzle ORM with row-level security keyed to JWT workspace claims, raw email and attachments live in S3-compatible object storage, and Typesense powers mailbox and thread search.