scorpiox-server-email — SCORPIOX CODE

Overview

scorpiox-server-email is a self-contained email server compiled from a single C source file (scorpiox/scorpiox-server-email.c). It implements a full SMTP Mail Transfer Agent on ports 25 and 587 (submission), an IMAP4rev1 server on port 993 (IMAPS), DKIM signing for outbound messages, Maildir-format storage, and email aliasing — all using mbedTLS for encryption with zero external dependencies.

Source: scorpiox/scorpiox-server-email.c | Type: server | TLS: mbedTLS

Protocol Support

SMTP — Simple Mail Transfer Protocol

Full MTA implementation supporting both relay (port 25) and submission (port 587). Handles EHLO/HELO handshake, MAIL FROM, RCPT TO, DATA, and QUIT commands. STARTTLS upgrade is supported for encrypted submission.

# SMTP session example
telnet mail.example.com 25
EHLO client.example.com
250-mail.example.com
250-STARTTLS
250-AUTH PLAIN LOGIN
250 OK
STARTTLS
220 Ready to start TLS
AUTH PLAIN AG1lQHNjb3JwaW94Lm5ldABwYXNzd29yZA==
235 Authentication successful

IMAP4rev1 — Internet Message Access Protocol

IMAP server on port 993 (IMAPS — TLS from connection start). Supports standard IMAP commands: LOGIN, SELECT, EXAMINE, FETCH, SEARCH, STORE, COPY, LIST, LSUB, SUBSCRIBE, UNSUBSCRIBE, and LOGOUT. Mail is stored in Maildir format and served directly from disk.

# IMAP session example (over TLS on port 993)
openssl s_client -connect mail.example.com:993
a1 LOGIN user@example.com password
a1 OK LOGIN completed
a2 SELECT INBOX
* 42 EXISTS
* 0 RECENT
a2 OK [READ-WRITE] SELECT completed
a3 FETCH 1:* (FLAGS ENVELOPE)
a4 LOGOUT

TLS — Transport Layer Security

All TLS operations use mbedTLS (embedded in the binary, no OpenSSL dependency). SMTP supports STARTTLS upgrade on port 587. IMAP uses TLS-from-connect on port 993. Certificates and keys are configured via EMAIL_TLS_CERT and EMAIL_TLS_KEY in scorpiox-env.txt.

Features

📬

Full SMTP MTA

Complete Mail Transfer Agent on port 25 (relay) and port 587 (submission) with STARTTLS support.

📥

IMAP4rev1 Server

Full IMAP server on port 993 (IMAPS). TLS from connection start. Standard mailbox operations.

🔏

DKIM Signing

Outbound messages are DKIM-signed automatically. Improves deliverability and prevents spoofing.

📁

Maildir Storage

Industry-standard Maildir format. One file per message. No database, no corruption on crash.

🔐

AUTH PLAIN/LOGIN

Standard SMTP authentication mechanisms. Credentials verified against the accounts file.

📧

Email Aliasing

Route messages with aliases. Forward addresses to other accounts without separate mailboxes.

🔄

Mail Queue with Retry

Outbound messages are queued and retried automatically on temporary delivery failures.

🔍

Email Filtering

Server-side filtering rules for incoming messages. Route, tag, or reject based on headers.

🖥️

TUI & Daemon Modes

Run interactively with a terminal UI or daemonize for production. PID file management included.

👤

Account Management

File-based account store. Add, remove, and manage email accounts from the CLI.

🔒

mbedTLS Encryption

All TLS via mbedTLS — compiled in, no OpenSSL, no GnuTLS, no shared library dependencies.

⚡

Zero Dependencies

Single C file, single binary. No Postfix, no Dovecot, no Exim. Just compile and run.

Configuration

All settings go in scorpiox-env.txt in your project root. The server reads these at startup.

Key	Default	Description
EMAIL_SMTP_PORT	25	SMTP MTA listening port for inbound relay
EMAIL_SUBMISSION_PORT	587	SMTP submission port (authenticated, STARTTLS)
EMAIL_IMAP_PORT	993	IMAPS port (TLS from connection start)
EMAIL_DOMAIN	—	Primary mail domain (e.g., scorpiox.net)
EMAIL_MAILDIR	./maildir	Root directory for Maildir storage
EMAIL_TLS_CERT	—	Path to TLS certificate (PEM format)
EMAIL_TLS_KEY	—	Path to TLS private key (PEM format)
EMAIL_ACCOUNTS_FILE	./accounts.txt	File containing user accounts (user:password_hash per line)

Example scorpiox-env.txt

# scorpiox-server-email configuration
EMAIL_DOMAIN=scorpiox.net
EMAIL_SMTP_PORT=25
EMAIL_SUBMISSION_PORT=587
EMAIL_IMAP_PORT=993
EMAIL_MAILDIR=/var/mail/scorpiox
EMAIL_TLS_CERT=/etc/ssl/certs/mail.scorpiox.net.pem
EMAIL_TLS_KEY=/etc/ssl/private/mail.scorpiox.net.key
EMAIL_ACCOUNTS_FILE=/etc/scorpiox/email-accounts.txt

Usage Examples

Start the email server (daemon mode)

# Start scorpiox-server-email as a daemon
sx server-email --daemon

# Start in foreground with TUI
sx server-email

# Check server status
sx server-email --status

Account management

# Add a new email account
sx server-email --add-account user@scorpiox.net

# List all accounts
sx server-email --list-accounts

# Remove an account
sx server-email --remove-account user@scorpiox.net

Send a test email (using scorpiox-email client)

# Send via the local server
sx email --to admin@scorpiox.net \
    --subject "Test from scorpiox-server-email" \
    --body "SMTP server is running."

# Send with HTML body from file
sx email --to admin@scorpiox.net \
    --subject "Report" \
    --html-file report.html

DNS records required

# MX record — route mail to your server
scorpiox.net.    IN  MX  10  mail.scorpiox.net.

# A record for the mail server
mail.scorpiox.net.  IN  A   203.0.113.10

# SPF — authorize the server to send
scorpiox.net.    IN  TXT "v=spf1 a:mail.scorpiox.net ~all"

# DKIM — public key for signature verification
sx._domainkey.scorpiox.net.  IN  TXT "v=DKIM1; k=rsa; p=MIGfMA0..."

# DMARC — policy
_dmarc.scorpiox.net.  IN  TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@scorpiox.net"

Architecture

┌─────────────────────────────────────────────────────────────┐ │ scorpiox-server-email │ │ (single C binary) │ ├──────────────┬──────────────────┬───────────────────────────┤ │ SMTP MTA │ SMTP Submission │ IMAP4rev1 Server │ │ Port 25 │ Port 587 │ Port 993 (IMAPS) │ │ Relay inbound │ STARTTLS + AUTH │ TLS-from-connect │ ├──────────────┴──────────────────┴───────────────────────────┤ │ │ │ ┌──────────┐ ┌───────────┐ ┌──────┐ ┌──────────────┐ │ │ │ DKIM │ │ Mail Queue│ │ Auth │ │ Email Filter │ │ │ │ Signing │ │ + Retry │ │ PLAIN│ │ Rules Engine │ │ │ └──────────┘ └───────────┘ │ LOGIN│ └──────────────┘ │ │ └──────┘ │ ├─────────────────────────────────────────────────────────────┤ │ mbedTLS — all TLS operations │ ├─────────────────────────────────────────────────────────────┤ │ Maildir (one file per message on disk) │ │ ~/maildir/user/cur/ ~/maildir/user/new/ .../tmp/ │ └─────────────────────────────────────────────────────────────┘

Security & TLS

✓ STARTTLS on port 587 — encrypted submission prevents credential sniffing
✓ TLS-from-connect on port 993 — IMAP connections are encrypted from the first byte
✓ mbedTLS built-in — no dependency on system OpenSSL; consistent behavior across platforms
✓ AUTH PLAIN/LOGIN — only accepted over TLS-encrypted connections
✓ DKIM signing — outbound messages carry a cryptographic signature for domain verification
✓ File-based accounts — no database attack surface; password hashes stored in a single file
✓ Maildir isolation — each user has separate directory; no shared mbox corruption risk
✓ Single static binary — minimal attack surface, easy to audit, no shared library hijacking

Related: scorpiox-email (Client)

scorpiox-server-email is the server. The corresponding client is scorpiox-email — a lightweight SMTP client built on libcurl that sends mail through any SMTP server, including scorpiox-server-email.

Client Config Key	Description
SMTP_HOST	SMTP server hostname (e.g., mail.scorpiox.net)
SMTP_PORT	SMTP port (default: 587)
SMTP_USER	Authentication username
SMTP_PASS	Authentication password
SMTP_FROM	Sender address
SMTP_TLS	TLS mode: starttls, ssl, or none