legacy-email-proxy/README.md

# Legacy Email Proxy

Proxy an unauthenticated, unencrypted POP3 / SMTP server to authenticated IMAPS and SMTPS backends.

## Features

- Exposes legacy `POP3` on `0.0.0.0:110` and legacy `SMTP` on `0.0.0.0:25`
- Forwards POP3 mailbox access to an IMAP backend
- Forwards SMTP submissions to an SMTPS backend
- Backend host, ports, and credentials are configured via environment variables

## Environment Variables

- `POP3_BIND_ADDR` (default `0.0.0.0`)
- `POP3_BIND_PORT` (default `110`)
- `SMTP_BIND_ADDR` (default `0.0.0.0`)
- `SMTP_BIND_PORT` (default `25`)

- `BACKEND_IMAP_HOST`
- `BACKEND_IMAP_PORT` (default `993`)
- `BACKEND_IMAP_USER`
- `BACKEND_IMAP_PASS`
- `BACKEND_IMAP_USE_SSL` (default `true`)
- `BACKEND_IMAP_USE_STARTTLS` (default `false`)

- `BACKEND_SMTP_HOST`
- `BACKEND_SMTP_PORT` (default `465`)
- `BACKEND_SMTP_USER`
- `BACKEND_SMTP_PASS`
- `BACKEND_SMTP_USE_SSL` (default `true`)
- `BACKEND_SMTP_USE_TLS` (default `false`)

- `BACKEND_MUTATE` (default `false`) - when `true`, POP3 deletions are propagated to the
  backend IMAP server (STORE +FLAGS / EXPUNGE). Default behaviour is to never mutate
  the backend mailbox on POP client deletions; the proxy only hides messages for the
  duration of the client session.

## Build and run

This project targets the latest Python LTS release. The included `Dockerfile` uses `python:3.12-slim`, which is compatible with Python 3.12 and later LTS releases.

```bash
docker build -t legacy-email-proxy .
docker run --rm -p 110:110 -p 25:25 \
  -e BACKEND_IMAP_HOST=imap.example.com \
  -e BACKEND_IMAP_PORT=993 \
  -e BACKEND_IMAP_USER=imap-user \
  -e BACKEND_IMAP_PASS=imap-pass \
  -e BACKEND_SMTP_HOST=smtp.example.com \
  -e BACKEND_SMTP_PORT=465 \
  -e BACKEND_SMTP_USER=smtp-user \
  -e BACKEND_SMTP_PASS=smtp-pass \
  legacy-email-proxy
```

The project is also a standard Python package (`pyproject.toml`), so it
installs without Docker. This puts a `legacy-email-proxy` command on `PATH`:

```bash
pip install .
legacy-email-proxy
```

`requirements.txt` remains the single source of runtime dependencies;
`pyproject.toml` reads it, so the Docker build and the package cannot drift.

## Nix

The repository is a flake. It exposes the package, an overlay, and a NixOS
module.

```bash
nix run .#legacy-email-proxy   # run it
nix build .#legacy-email-proxy # build it; the test suite runs as part of the build
nix develop                    # dev shell with pytest
```

As a NixOS service, with the proxy's own flake as an input:

```nix
{
  inputs.legacy-email-proxy.url = "git+https://code.emmathe.dev/lyrathorpe/legacy-email-proxy";
  # optionally: inputs.legacy-email-proxy.inputs.nixpkgs.follows = "nixpkgs";
}
```

```nix
{
  imports = [ inputs.legacy-email-proxy.nixosModules.default ];

  services.legacy-email-proxy = {
    enable = true;
    settings = {
      POP3_BIND_ADDR = "10.0.0.1";
      SMTP_BIND_ADDR = "10.0.0.1";
      BACKEND_IMAP_HOST = "imap.example.com";
      BACKEND_IMAP_USER = "someone@example.com";
      BACKEND_SMTP_HOST = "smtp.example.com";
      BACKEND_SMTP_USER = "someone@example.com";
    };
    # Credentials belong here, not in `settings` -- anything in `settings`
    # lands in the world-readable Nix store.
    environmentFile = "/var/lib/legacy-email-proxy/backend.env";
  };
}
```

`settings` accepts any of the environment variables listed above; booleans and
integers are converted for you. The service runs under `DynamicUser`, with
`CAP_NET_BIND_SERVICE` granted only while a listener is on a privileged port.
It opens no firewall ports — see "Security".

Prefer to manage the package yourself? `overlays.default` provides
`pkgs.legacy-email-proxy`, and `nixosModules.legacy-email-proxy` is the same
module without the package default wired to this flake.

## Tests

Install development dependencies and run the test suite:

```bash
python -m venv .venv
source .venv/bin/activate
python -m pip install --upgrade pip
pip install -r requirements-dev.txt
pytest -q
```

## Notes

This implementation begins the proxy with a minimal POP3 command set and SMTP delivery path. It is designed to start development on the required application architecture.

## Security

By design, the front-end POP3 (port 110) and SMTP (port 25) listeners are **unencrypted** and **unauthenticated**. Anyone who can reach port 110 obtains full mailbox access, and anyone who can reach port 25 can relay mail through the configured backend SMTP credentials, which is an open relay from the network's perspective.

Because of this, the listeners **must** be bound to a trusted internal network only, such as a private Docker bridge, a VPN interface, or localhost, and **must not** be exposed to untrusted networks or the public internet.

Operators who need to restrict the bind address can set `POP3_BIND_ADDR` / `SMTP_BIND_ADDR` to a specific internal interface instead of `0.0.0.0`.