What Is an Access Control List (ACL)?

What is an Access Control List (ACL)?

An Access Control List (ACL) is a set of rules that define which users, systems, or processes are allowed to access specific resources, and what actions they are permitted to perform.

In the context of email and network security, ACLs are used to control who can connect to mail servers, relay messages, or interact with particular services. By specifying allowed or denied IP addresses, domains, or users, administrators can protect infrastructure from unauthorized access and misuse.

ACLs are a foundational part of security policy enforcement across firewalls, routers, and mail systems.

How Access Control Lists Work

An ACL acts like a digital gatekeeper. Each entry (or “rule”) in the list contains conditions that determine whether access is granted or denied. For example, in a mail server configuration, an ACL might specify:

• Allow outbound mail from internal IP ranges (192.168.0.0/16)
• Deny relay access from external networks
• Permit connections only on specific ports (e.g., 25 or 587)

When a connection request arrives, the server checks the ACL entries in order until one matches. Depending on the match, the connection is either accepted, rejected, or logged for further review.

ACLs can apply to various layers:

• Network level: controlling IP routing or firewall access
• System level: defining which users or groups can read, write, or execute files
• Application level: limiting who can use specific features, like sending bulk mail or accessing APIs

Why ACLs Are Important for Email Security

Properly configured ACLs prevent unauthorized mail relay, spam abuse, and malicious access to SMTP services.

Without ACLs, attackers could attempt to connect directly to your mail infrastructure to send spam, test credentials, or exploit vulnerabilities. Poor ACL hygiene is often the first step toward an open relay or compromised mail server.

Key benefits of strong ACL implementation include:

• Blocking unauthorized IPs before they reach the mail system
• Reducing spam and phishing attempts at the server level
• Protecting bandwidth and server resources from abuse
• Providing granular control over who can send or relay messages

In short, ACLs serve as the first line of defense, stopping unwanted traffic before deeper security checks (like SPF, DKIM, or DMARC) take place.

ACLs and DMARCeye

While DMARCeye focuses on email authentication and visibility, ACLs play a complementary role by securing the transport layer, i.e., the infrastructure that handles email before authentication is verified.

If a server lacks ACL protections, unauthorized IPs may generate traffic that appears in your DMARC aggregate reports. DMARCeye helps identify these patterns by showing:

• Which IPs are sending unauthenticated mail
• How frequently unauthorized sources attempt delivery
• Whether configuration issues align with infrastructure access problems

By using ACLs alongside DMARCeye’s visibility, organizations can reduce noise in reports, improve enforcement accuracy, and prevent abuse at the source.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.