Mox: Modern Go-Based Mail Server

Mox is a modern, open-source mail server written in Go that aims to make self-hosting email straightforward. It implements SMTP, IMAP, SPF, DKIM, DMARC, DANE, MTA-STS, autoconfig, and a webmail interface — all in a single binary with automatic TLS. Mox represents the latest generation of mail server software with security-first design and operational simplicity.

Why Mox?

• Single binary — one Go binary handles everything: SMTP, IMAP, webmail, admin
• Security by default — DANE, MTA-STS, strict TLS, automatic DKIM
• Built-in webmail — modern, fast webmail interface included
• Junk filter — Bayesian spam filter with reputation tracking
• Easy setup — interactive quickstart generates all DNS records
• Low resource usage — runs well on 512MB RAM

Installation

# Download latest release
MOX_VERSION=v0.0.11
wget https://github.com/mjl-/mox/releases/download/${MOX_VERSION}/mox-${MOX_VERSION}-linux-amd64.tar.gz
tar xzf mox-${MOX_VERSION}-linux-amd64.tar.gz
sudo mv mox /usr/local/bin/
sudo chmod +x /usr/local/bin/mox

Initial Setup

# Run the quickstart (generates configuration and DNS records)
sudo mox quickstart admin@example.com

# This creates:
# - config/mox.conf (main configuration)
# - config/domains.conf (domain configuration)
# - DKIM keys
# - List of required DNS records

# Review and apply the suggested DNS records before starting

Configuration

# config/mox.conf (generated by quickstart, customize as needed)
DataDir: data
Hostname: mail.example.com
AdminPasswordFile: adminpassword

Listeners:
    public:
        IPs:
            - 0.0.0.0
        Hostname: mail.example.com
        TLS:
            ACME: letsencrypt
        SMTP:
            Enabled: true
        Submission:
            Enabled: true
        IMAP:
            Enabled: true
        IMAPS:
            Enabled: true
        AutoconfigHTTPS:
            Enabled: true
        WebmailHTTPS:
            Enabled: true
        AdminHTTPS:
            Enabled: true

Postmaster:
    Account: admin
    Mailbox: Postmaster

DNS Records

The quickstart generates all required records. Apply them in your DNS provider:

# Generated by mox quickstart — example output:
mail.example.com.       A       203.0.113.1
example.com.            MX  10  mail.example.com.
example.com.            TXT     "v=spf1 mx -all"

# DKIM (mox generates multiple selectors)
20250115._domainkey.example.com.    TXT    "v=DKIM1; k=ed25519; p=..."
20250115e._domainkey.example.com.   TXT    "v=DKIM1; k=rsa; p=..."

# DMARC
_dmarc.example.com.     TXT     "v=DMARC1; p=reject; rua=mailto:dmarc@example.com"

# MTA-STS
_mta-sts.example.com.   TXT     "v=STSv1; id=20250115T000000Z"

# DANE (TLSA records)
_25._tcp.mail.example.com.  TLSA    "3 1 1 abc123..."

# Autoconfig/Autodiscover
_imaps._tcp.example.com.    SRV     0 1 993 mail.example.com.
_submission._tcp.example.com. SRV   0 1 587 mail.example.com.

Managing Accounts

# Add a new account
mox config account add user@example.com

# Set password
mox config setaccountpassword user@example.com

# List accounts
mox config account list

# Add an email alias
mox config alias add info@example.com user@example.com

Systemd Service

# /etc/systemd/system/mox.service
[Unit]
Description=Mox Mail Server
After=network.target

[Service]
Type=notify
ExecStart=/usr/local/bin/mox serve
WorkingDirectory=/opt/mox
Restart=on-failure
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true
ProtectSystem=strict
ReadWritePaths=/opt/mox/data

[Install]
WantedBy=multi-user.target

Web Interfaces

• Webmail: https://mail.example.com/webmail/ — full-featured webmail for users
• Admin: https://mail.example.com/admin/ — server administration, queue management, logs
• Account: https://mail.example.com/account/ — user self-service (password change, device security)

Junk Mail Filtering

Mox includes a Bayesian junk filter that learns from user behavior:

• Moving messages to Junk trains the filter as spam
• Moving messages from Junk to Inbox trains as ham
• The filter uses per-account reputation tracking
• SPF, DKIM, and DMARC results influence the junk score

Monitoring and Maintenance

# Check server status
mox config check

# View mail queue
mox queue list

# Retry queued messages
mox queue retry

# View delivery logs via admin interface
# https://mail.example.com/admin/ → Delivery logs

# Check TLS certificate status
mox config checkdomain example.com

Backup

# Mox stores everything in the data directory
# Back up the entire mox directory
tar czf /backup/mox-$(date +%Y%m%d).tar.gz /opt/mox/

# Or use mox built-in backup
mox backup /backup/mox-$(date +%Y%m%d)/

Best Practices

• Use the quickstart to generate configuration and DNS records — it handles complex setups automatically
• Enable DANE (TLSA records) for the strongest transport security
• Mox signs outgoing email with both Ed25519 and RSA DKIM keys for maximum compatibility
• Set DMARC policy to reject once you have verified email authentication works correctly
• Monitor the admin dashboard for delivery issues and junk filter effectiveness
• Keep Mox updated — it is actively developed with security and feature improvements
• Use the built-in autoconfig for easy email client setup