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Simple Mail Transfer Protocol (SMTP)

This manual aims at providing a guide to harden the implementation of SMTP (Simple Mail Transfer Protocol) on your system. SMTP is a protocol for sending e-mail messages between servers.

Disable Unnecessary Services​

  • Turning off unnecessary or unused services can save resources and reduce vulnerabilities that can be exploited.
systemctl disable serviceName
systemctl stop serviceName

Replace serviceName with the name of the service you want to disable.

Update Latest Security Patches​

  • Always maintain the MTA updated with the latest security patches.
apt-get update
apt-get upgrade

Limit SMTP port​

  • Make sure that the SMTP port (25) is inaccessible from outside.
iptables -A INPUT -p tcp --dport 25 -j DROP

Require Authentication​

  • Require SMTP authentication for outbound relay. All legitimate users and applications are authenticated before being allowed to relay messages to other domains.

In file /etc/postfix/main.cf, locate and modify the smtpd_relay_restrictions parameter:

smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination

Strong Password Policies​

  • Apply strong password policies to all accounts.

Secure SSL/TLS Configuration​

  • Prefer secured connections when possible, make sure you configure SMTP connections over TLS/SSL.

In file /etc/postfix/main.cf, add in this line:

smtpd_tls_security_level = may
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_security_level = may
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3

This will allow secured transport only, and disable deprecated SSL versions.

SPF, DKIM and DMARC​

  • Implement Sender Policy Framework (SPF), Domain Keys Identified Mail (DKIM) and Domain-based Message Authentication, Reporting and Conformance (DMARC) policies for the domains.

Limit Rate of Emails​

  • Limiting the rate at which your server sends Mail/SMTP traffic can protect you from being blacklisted and also from possible outbound spam or virus issues.

In file /etc/postfix/main.cf

smtpd_client_message_rate_limit = 100

Monitoring and Logging​

  • Keep an eye on your server’s activity. Regularly check system and mail logs for any suspicious activity.