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lyrathorpe e51740b8db Document unauthenticated listener exposure (#13 )

Adds a Security section: the front-end POP3/SMTP listeners are unauthenticated and must be bound to a trusted internal network only. Closes #9

Reviewed-on: #13
Co-authored-by: Lyra Thorpe <iam@emmathe.dev>
Co-committed-by: Lyra Thorpe <iam@emmathe.dev>

2026-06-17 17:33:51 +01:00

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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)

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.

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

Tests

Install development dependencies and run the test suite:

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.

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