Postern ======= Subscribe to our mailing list by sending an email to `postern+subscribe@lists.p25.dev` for announcements of new versions. The mailing list server runs Postern and mlmmj. Postern is a modern all-in-one mail delivery agent. It combines IMAP (IMAP4rev2), SMTP Submission and SMTP delivery. We have EU Sovereignity at our core values. We welcome users and contributions from around the world and will keep service providers such as server hosting and DNS registrars EU based. > **⚠️ WARNING:** This software is in an experimental state. > Do not use in production or for any important emails while we work towards v1.0. Overview -------- What makes Postern special is its policy engine. Imagine it as a modern Sieve based on [Starlark](https://starlark-lang.org), which uses Python syntax. The [policy.star file](https://code.raphting.dev/postern.git/blob/main/policy.star) has an example how new senders end up in a "Gatekeeper" mailbox. Simply moving a message from one folder to another updates how the policy engine reacts to this sender for future messages. Reacting on side effects such as database state or network calls is probably the biggest advantage of the policy engine over Sieve when it comes to modern email management. Postern has secure defaults, stores emails encrypted at rest and runs as a single binary with an SQLite backend. This simplifies backups. All you need to back up is your `/var/lib/postern` (depending on your configuration) directory. You can subscribe to *announcements* by sending an email to `postern+subscribe@lists.p25.dev`. The mailing list server runs on Postern and mlmmj. Architecture ------------ Postern was developed for easy email self-hosting while not fighting email deliverability or a multitude of DNS records. Postern is designed to sit behind an email relay and MX backup like [p25.dev](https://p25.dev). With just 3 DNS records (DKIM, MX and domain verification) you are ready for sending and receiving emails. Postern can run behind other services such as Postfix too, as it is fully standards compliant. The main use case will be the operation behind [p25.dev](https://p25.dev) though just for its simplicity and to support more inexperienced admins in self-hosting email. Your emails are stored encrypted in [Data At Rest Encryption (DARE) format](https://github.com/minio/sio). Note that the SQLite database stores email _metadata_ such as To and From addresses and Subjects in **plain text** to support fast IMAP search requests. All networking is secure by default. IMAP and SMTP Submission run on implicit TLS. SMTP Delivery requires STARTTLS and authentication. Passwords are generated by Postern to avoid weak choices. DKIM key management is designed for easy key rotation. Quickstart ---------- Postern is in alpha state. You might experience some trial and error to arrive at a working solution. This quickstart tutorial assumes you work on a modern Linux installation. Because you read this Readme, it looks like you have access to the rest of the source code and can compile it with `go build -o postern .`. 1. Install the `postern.service` file under `/etc/systemd/system/postern.service`. 2. Place `policy.star` under `/etc/postern/policy.star` and adjust to your requirements. 3. For secure storage of the masterkey, run `systemd-ask-password -n | systemd-creds encrypt - /etc/credstore/masterkey` 4. Retrieve a TLS certificate, for example with [acme.sh](https://github.com/acmesh-official/acme.sh). Make sure the systemd service can access the certificates (`/etc/postern/`). 5. Enable and start Postern `systemctl enable postern` and `systemctl start postern` 6. Use the Postern CLI to add configuration. Based on the systemd integration, this is admittedly clunky and will be improved soon. When running the Postern CLI, it will communicate with the **running** Postern daemon over a unix socket. The used socket is logged during startup and can be configured with environment variable `POSTERN_SOCKET`. ```Shell export POSTERN_SOCKET=/run/postern/postern.sock /usr/local/bin/postern ``` ### CLI admin access Unix sockets have an Owner and a Group like any other file on Unix or Linux. Instead of creating an admin user and a password in Postern, access is guarded by existing Unix file permissions. *root* should always have access. The systemd Postern user also has access, even though in the default configuration it is dynamically created by systemd. If you want to use a non-root and non-Postern user for the CLI, have the systemd service run under a specific group that your non-root user also belongs to. ### First configuration steps (example) You will need to create a user, a backup-mx user for email delivery and a DKIM key. ```Shell postern user add testuser postern user add-address testuser user@example.com postern backup-mx add p25dev postern dkim add selector example.com # Don't forget to enable the key with 'dkim enable selector example.com' ``` If you want a smooth start for sending and receiving emails, make an account at [p25.dev](https://p25.dev) to integrate Postern with it. Contributing via Mailing List ----------------------------- We manage development via plain-text email patches rather than web-based pull requests. **Mailing list:** `postern-dev@lists.p25.dev` (subscribe with an email to `postern-dev+subscribe@lists.p25.dev`) Use `git send-email` to contribute your patch. For a tutorial see [https://git-send-email.io](https://git-send-email.io/). It is okay to use the mailing list as a chat rather than a formalized procedure. Just say 'hi'! > **💡 TIP:** You can integrate `git send-email` with Postern too! Security -------- Please do not disclose security vulnerabilities via the public mailing list. Instead, use `https://p25.dev/.well-known/security.txt`