smtp-gateway 1.0.0

SMTP gateway API with PII column scanning, CIDR allowlist, and optional ADLS audit

SMTP Gateway (smtp-gateway)

Flask API that sends mail through an internal SMTP relay after checking caller IP (CIDR), bearer token, recipient domains, and PII column names in HTML bodies and CSV/HTML attachments. Optional audit to Azure ADLS and notification emails.

Install

Git is not required. See PUBLISHING.md.

From PyPI (after you publish)

pip install "smtp-gateway==1.0.0"

On VM without PyPI (copy wheel)

pip install /path/to/smtp_gateway-1.0.0-py3-none-any.whl

From project folder (no git, no PyPI)

pip install .

Local development

pip install -e .

Required configuration

Copy .env.example to .env on the server and set at least:

Variable	Purpose
API_TOKEN	Clients send Authorization: Bearer <token>
FROM_EMAIL	Gateway sender address
SMTP_SERVER	Internal SMTP host
ALLOWED_DOMAINS	Comma-separated recipient domains

Startup fails if any required variable is missing.

PII blocklist and caller CIDRs

The package ships example JSON under smtp_gateway/data/:

• blocklist.json — sample column names
• allowed_cidrs.json — empty list (blocks all IPs until you configure)

For production, point to your own files (recommended):

export BLOCKLIST_PATH=/etc/smtp-gateway/blocklist.json
export ALLOWED_CIDRS_PATH=/etc/smtp-gateway/allowed_cidrs.json

Format: JSON arrays. Blocklist entries are strings (one column name per entry). CIDRs are strings like "10.0.0.0/24".

Without ALLOWED_CIDRS_PATH, bundled allowed_cidrs.json is used — empty [] means every caller gets 403 Forbidden IP.

Optional

Variable	Purpose
SECURITY_ALERT_EMAIL / SECURITY_ALERT_CC	Email on blocked requests
AUDIT_NOTIFICATION_EMAIL / AUDIT_NOTIFICATION_CC	Audit copy per send/block
ADLS_ACCOUNT_NAME, ADLS_CONTAINER_NAME, MANAGED_IDENTITY_CLIENT_ID	ADLS audit (skipped if unset)
GUNICORN_WORKERS, BIND	Process count and listen address

Run

Production (HA — use behind a load balancer, 2+ VMs):

set -a && source .env && set +a
smtp-gateway

Equivalent:

gunicorn -w 4 -b 0.0.0.0:5000 'smtp_gateway.app:create_app()'

Local development (Flask dev server):

set -a && source .env && set +a
python -m smtp_gateway

Health check: GET /health → {"status":"healthy"}.

API

POST /send-mail (multipart form):

Field	Required	Notes
to	yes	Recipient(s)
subject	yes
body	yes	HTML (table headers scanned for PII)
cc	no
file	no	.csv, .html, .htm only

Headers:

• Authorization: Bearer <API_TOKEN>
• Caller IP must match allowed_cidrs.json

Client checklist

• Call from an allowed CIDR
• Use allowed recipient domains
• Avoid blocklisted column names in HTML tables and CSV/HTML attachments

Package layout

src/smtp_gateway/
  app.py              create_app()
  config.py           environment settings
  routes/             /health, /send-mail
  security/           CIDR, domains, auth
  pii/                extract, blocklist, matcher
  audit/              ADLS, email notifications
  data/               bundled example JSON

What you supply vs what is bundled

Bundled in package	You supply on the VM
PII matching code	Full blocklist.json via BLOCKLIST_PATH
Example blocklist	Real allowed_cidrs.json via ALLOWED_CIDRS_PATH
Flask routes, audit code	.env secrets and SMTP/ADLS access
smtp-gateway CLI	Load balancer, systemd, managed identity + RBAC