Secure Your Emails: Best Practices for Dwarf Mail ServerDwarf Mail Server is a compact, resource-efficient mail server that appeals to small networks, home labs, and organizations wanting a minimal, manageable mail stack. Small footprint and simplicity are strengths, but they also mean administrators must be deliberate about security decisions. This guide covers practical, actionable best practices to secure your Dwarf Mail Server deployment — from hardening the host and the mail stack to authentication, transport protection, spam defense, monitoring, and incident response.
1. Secure the host environment
A secure mail server begins with a secure host.
- Keep the OS and packages updated. Apply security patches promptly. Use automated patching where appropriate but test critical updates in staging for production systems.
- Run the mail server as an unprivileged user. Ensure Dwarf processes don’t run as root; restrict filesystem access to the minimum required.
- Minimize installed software. Remove unneeded packages and services to shrink the attack surface.
- Use a host-based firewall. Only permit necessary ports (typically 25 for SMTP, 587 for submission, 465 for SMTPS if used, and ⁄993 or ⁄995 for IMAP/POP if the server provides them). Block or rate-limit other traffic.
- Enable automatic intrusion detection. Tools like fail2ban can block repeated failed connections against the SMTP/authentication ports.
- Harden SSH and remote access. Use key-based authentication, change default ports if desired, disable password auth, and consider IP allowlists or VPN-only management access.
2. Enforce strong authentication and access control
Authentication prevents unauthorized use and relay.
- Require authentication for submission (port 587). Prevent open relay by disabling unauthenticated mail submission for clients.
- Use strong password policies. Enforce minimum length, complexity, and rotation policies for user passwords. Prefer passphrases.
- Support multi-factor authentication (MFA) for admin accounts. If Dwarf’s admin interface supports MFA, enable it. If not, protect the admin host with MFA via VPN or jump host.
- Limit admin access by network. Restrict management interfaces to specific IPs or ranges.
- Audit accounts and remove unused mailboxes. Periodically review and disable stale accounts.
3. Encrypt transport with TLS
Protect messages in transit to prevent interception and downgrade attacks.
- Enable STARTTLS and/or SMTPS. Configure Dwarf Mail Server to support TLS for SMTP submission and relay.
- Use valid, automated certificates. Obtain certificates from a trusted CA — Let’s Encrypt is a common free option — and automate renewal (certbot or ACME clients).
- Prefer modern TLS settings. Disable TLS 1.0 and 1.1; enable TLS 1.2 and 1.3 only. Use strong cipher suites and enable forward secrecy (ECDHE).
- Enforce opportunistic TLS with strict options for sensitive peers. For internal or partner mail relays, consider enforcing TLS (MTA-STS, DANE where supported).
- Monitor certificate expiry. Set alerts for impending certificate expiration to avoid downtime.
4. Authenticate and validate mail: SPF, DKIM, DMARC
These standards reduce spoofing, phishing, and improve deliverability.
- Publish an SPF record. Create an SPF TXT record listing authorized sending servers for your domain to help receiving MTAs reject unauthorized senders.
- Sign outgoing mail with DKIM. Configure Dwarf or a signing proxy to attach DKIM signatures. Use a 2048-bit or stronger key, publish the public key in DNS, and rotate keys periodically.
- Publish a DMARC policy. Start with a relaxed policy (p=none) to collect reports, then move to quarantine or reject (p=quarantine / p=reject) once you’ve verified legitimate sending sources.
- Aggregate and forensic reporting. Enable DMARC reporting to monitor abuse and misconfigurations. Use tools to parse and act on reports.
- Keep DNS secure. Use DNSSEC and restrict zone changes to authorized administrators.
5. Anti-spam and content filtering
Prevent spam and malware from entering or leaving your infrastructure.
- Deploy spam filtering. Use a combination of reputation, content analysis, and DNSBLs. If Dwarf lacks built-in filtering, place a filtering proxy (SpamAssassin, rspamd) in front.
- Scan attachments for malware. Integrate antivirus (ClamAV or commercial engines) on the mail path to scan attachments.
- Use greylisting selectively. Greylisting can reduce spam but may delay legitimate mail; apply it where acceptable.
- Rate-limit outgoing mail. Prevent compromised accounts from sending mass spam by throttling per-user and per-IP sending rates.
- Quarantine suspicious messages. Store questionable messages for admin review rather than outright delivering them.
6. Secure mail storage and access
Protect messages at rest and user access methods.
- Encrypt mail storage. Use disk-level encryption (LUKS) for server disks or application-level encryption when possible to protect data if disks are stolen.
- Protect IMAP/POP access with TLS. Require encrypted client connections (IMAPS/POP3S) and disable plaintext logins over unencrypted channels.
- Use secure webmail. If providing webmail, ensure the webmail software is up-to-date and runs with least privilege. Host webmail behind WAF or reverse proxy with TLS.
- Implement mailbox quotas and limits. Prevent disk exhaustion attacks by setting per-user quotas and alerts.
7. Relay, rate limits, and outbound policies
Control how mail leaves your network to avoid blacklisting.
- Avoid open relay. Ensure relaying is restricted to authenticated users or authorized networks.
- Apply per-account and per-connection rate limits. This prevents abuse if credentials are stolen.
- Monitor outgoing reputation. Track bounce rates, complaints, and blacklists. Configure feedback loops where available with major providers.
- Use proper HELO/EHLO, PTR, and reverse DNS. Set a matching PTR record for your mail server IP and ensure HELO identifies the correct hostname.
8. Logging, monitoring, and alerting
Visibility is essential for detecting abuse and incidents.
- Enable detailed logging. Log connections, authentication attempts, deliveries, and errors. Rotate logs securely.
- Centralize logs. Send logs to a central syslog, SIEM, or logging service for retention, correlation, and analysis.
- Monitor key metrics. Track authentication failures, queue size, delivery delays, bounce rates, and unusual spikes in outbound mail.
- Alert on anomalies. Create alerts for high failure rates, sudden outgoing volume, or blacklisting notifications.
- Perform periodic audits. Review logs and configuration changes, and keep an audit trail for administrative actions.
9. Backup and recovery
Prepare for data loss and compromise.
- Back up mailboxes and configuration. Include both user data and server configuration in scheduled, tested backups.
- Encrypt backups and restrict access. Store backups offsite and ensure they are protected with strong encryption and access controls.
- Test restore procedures. Regularly restore backups to verify integrity and recovery time.
- Have an incident response plan. Define steps for key compromise, spam outbreaks, data breaches, and DNS hijacking scenarios.
10. Stay informed and maintain best practices
Security is ongoing.
- Subscribe to security advisories. Follow Dwarf Mail Server updates, relevant Linux distro advisories, and major email ecosystem alerts.
- Regularly review and update configurations. Re-evaluate TLS, authentication methods, and filtering rules at least quarterly.
- Train users. Teach users to recognize phishing, use strong passwords, and report suspicious mail.
- Consider external audits. Periodic security assessments or penetration tests can reveal blind spots.
Example hardening checklist (quick reference)
- OS updates applied and unnecessary services removed
- Mail processes run unprivileged
- Firewall permits only required ports
- STARTTLS/SMTPS enabled with Let’s Encrypt certificates
- SPF, DKIM, DMARC configured and monitored
- Spam and malware filtering in place (rspamd/SpamAssassin + ClamAV)
- Rate limits and relaying restrictions enforced
- Encrypted mail storage and encrypted client access (IMAPS/POP3S)
- Centralized logging, alerts for anomalies
- Regular backups and tested restores
Securing Dwarf Mail Server requires combining host hardening, strict authentication, encryption in transit and at rest, proper anti-spam measures, vigilant monitoring, and an incident-ready backup strategy. Applied together, these practices will significantly reduce risk, improve deliverability, and keep your email infrastructure resilient.
Leave a Reply