SMTP250.org TECHNICAL WHITEPAPER

Version: 1.0 (November 2025)
Status: Production
Author: ADHOCON GmbH, Development Team

This document outlines the technical architecture, security measures, and forensic validity of the SMTP250.org proof-of-delivery service.

1. ARCHITECTURE OVERVIEW

SMTP250 operates as a Transactional Man-in-the-Middle (TMitM) Proxy specifically designed for legal logging.

[CLIENT] ---> [SMTP250 INGRESS] ---> [PROCESSOR] ---> [EGRESS RELAY] ---> [RECIPIENT] (SMTP) (Python aiosmtpd) (Hash/Sign) (Postfix) (MX Record)

1.1 Ingress (The Listener)

We utilize a custom Python-based SMTP handler binding to Port 25. It enforces Opportunistic TLS (STARTTLS). This ensures that the connection between the sender and our server is encrypted if the client supports it.

1.2 The Processor (Forensic Engine)

Upon receipt, the email is not stored in a persistent database. Instead:

The raw byte-stream is isolated in volatile memory (RAM).
A SHA-256 Hash is calculated over the entire raw content (Headers + Body + Attachments).
This ensures mathematical integrity: Changing a single bit in the original email would result in a completely different hash.

2. CRYPTOGRAPHY & SIGNING

The core value proposition is the cryptographically signed PDF ("The Receipt").

2.1 Key Management

Algorithm: RSA-2048 with SHA-256 digest.
Padding: PSS (Probabilistic Signature Scheme) for maximum security against collision attacks.
Key Storage: The private key is stored on a hardened server partition with restricted root-only access.

2.2 Public Key for Verification

To verify the signature on any generated PDF, you can use our Public Key:

3. EMAIL DELIVERABILITY & COMPLIANCE

3.1 SPF / DKIM / DMARC

To ensure high deliverability, SMTP250.org implements full sender authentication:

SPF: Strict allow-listing of our outbound IPs.
DKIM: Every outgoing email (receipts and relays) is signed with rsa-sha256 using OpenDKIM.
DMARC: Policy set to quarantine to prevent spoofing.

3.2 The Forwarding Challenge (SRS)

When relaying emails (e.g., from GMX to Gmail), standard forwarding breaks SPF alignment. SMTP250 solves this by rewriting the Envelope Sender to noreply@mail.smtp250.org while keeping the Header From intact. This ensures that technical bounces return to us for analysis, while the recipient still sees the original sender in their email client.

4. PRIVACY & GDPR (DSGVO)

We adhere to a strictly minimal data retention policy ("Privacy by Design").

No Database: Email bodies are processed in RAM and temporary files only.
Immediate Deletion: Once the transaction (payment & delivery) is complete, the temporary file is wiped from the disk.
Server Location: All processing occurs in Falkenstein/Nuremberg, Germany (Hetzner Online GmbH), fully under German jurisdiction.