When analyzing DMARC (Domain-based Message Authentication, Reporting, and Conformance) reports, it’s not uncommon to discover that IP addresses owned by large, reputable companies are being flagged for sending spam. This revelation might seem surprising, but the root cause often lies in compromised systems within their networks.
How Malware Leads to Spam Email
Hackers frequently exploit security vulnerabilities by infecting computers with malware, turning them into part of a botnet—a network of infected devices controlled remotely. These botnets are then leveraged to send spam emails globally, tarnishing the reputation of the affected IP addresses.
A compromised system sending spam can cause an organization's public IP address to become blacklisted, affecting legitimate email communications and the company’s overall credibility.
Tactics to Prevents Spam Email from Your Network
1. Configure Your Firewall to Block Outbound Port 25
A key strategy to prevent your network from inadvertently sending spam is to restrict outbound connections to port 25. Port 25 is traditionally used for SMTP (Simple Mail Transfer Protocol), which facilitates the sending of emails. While legitimate email servers use this port, so do malicious botnets attempting to distribute spam. This will prevent infected computers in your network from sending emails directly without authentication.
If you operate mail servers that need to send emails using port 25, explicitly allow outbound traffic from only these servers. Carefully manage these exceptions to ensure they are not exploited.
2. Strengthen Endpoint Security
- Deploy anti-malware solutions across all devices in your network to reduce the likelihood of malware infections.
- Conduct regular security audits and update software to patch vulnerabilities.
3. Monitor and Audit Network Traffic
- Regularly review DMARC reports to identify any unusual activity, such as unexpected email-sending patterns from unauthorized systems.
- Implement network monitoring tools to detect and address anomalies promptly.