Async SMTP Server (Rust)

A fully asynchronous SMTP server built in Rust implementing core RFC-compliant email protocol commands with TLS, Authentication, and Database-backed mail storage.

This project focuses on building a production-style SMTP server from scratch using the Rust async ecosystem.

Features

Core SMTP Commands

• EHLO / HELO
• MAIL FROM
• RCPT TO
• DATA
• RSET
• NOOP
• VRFY
• HELP
• QUIT

Security Features

• STARTTLS support
• AUTH LOGIN authentication
• TLS handshake using rustls
• Authenticated mail submission

Async Architecture

• Tokio async TCP server
• Concurrent multi-client handling
• Non-blocking database writes using spawn_blocking

Database (Diesel + SQLite)

• Email storage
• Multi-recipient support
• User authentication
• Mailbox listing

Tech Stack

• Rust
• Tokio
• Diesel ORM
• SQLite
• rustls
• r2d2 connection pooling
• Axum (API layer — WIP)

Architecture

Client
  │
  ▼
TCP Server (Tokio)
  │
  ▼
Session State Machine
  │
  ├── TLS Layer
  ├── Auth Layer
  └── SMTP Commands
  │
  ▼
Storage Layer (Diesel)
  │
  ▼
SQLite Database

Project Structure

src/
 ├── main.rs
 ├── session.rs
 ├── parser.rs
 ├── storage.rs
 ├── models.rs
 ├── schema.rs
 ├── tls.rs
 ├── config.rs
 ├── response.rs
 └── api/ (WIP)

Getting Started

1. Clone

git clone https://github.com/yourrepo/smtp-server
cd smtp-server

2. Setup Environment

Create .env file:

DATABASE_URL=emails.db

3. Generate TLS Certificates

mkdir certs

openssl req -x509 -newkey rsa:4096 \
-keyout certs/key.pem \
-out certs/cert.pem \
-days 365 -nodes

4. Run Server

cargo run

Server runs on:

• SMTP Server: 127.0.0.1:2525
• API Server (WIP): 127.0.0.1:3000

Example SMTP Flow

EHLO localhost
STARTTLS
AUTH LOGIN
MAIL FROM:<user@test.com>
RCPT TO:<receiver@test.com>
DATA
Hello world
.
QUIT

Authentication

Users stored in database:

• users table

Authentication flow:

• AUTH LOGIN
• Username
• Password

Database Schema

Tables:

• users
• emails
• recipients

Supports:

• Multi-recipient emails
• Mailbox listing
• Authentication

Current Status

Completed:

• SMTP Protocol
• Async TCP Server
• TLS Support
• Authentication
• Database storage
• Multi-recipient emails

In Progress:

• REST API
• UI Dashboard
• Attachments support

Planned:

• Message Queue
• DKIM/SPF verification
• SMTP relay support
• Production hardening

Goals

This project aims to:

• Learn the SMTP protocol deeply
• Build a production-grade Rust networking system
• Understand async Rust architecture
• Build mail infrastructure from scratch

Why This Project

Most developers use email services. This project builds one from scratch.

Focus Areas:

• Async Rust
• Protocol design
• Networking
• Database design
• Security (TLS + Auth)