Securing an Exim SMTP server is absolutely critical to prevent abuse, protect your reputation, and ensure reliable mail delivery. A misconfigured server is a prime target for spammers and malicious actors. Achieving a robust configuration requires careful attention to several key areas, moving beyond default settings which are often too permissive for production environments.
The first priority is implementing strict access controls. This is primarily managed through Exim’s Access Control Lists (ACLs). You must define rules for incoming connections (ACLSMTPCONNECT), incoming email headers and body (ACLDATA), and recipients (ACLRCPT). These ACLs should deny mail relays for unauthorized senders, validate sender and recipient addresses where possible, and reject connections or messages that fail checks like proper HELO/EHLO hostname or PTR record validation. A well-structured ACL setup is the cornerstone of Exim security.
Next, authentication is vital, particularly for legitimate users sending mail through your server (SMTP AUTH). Always require authentication for sending mail from external networks. Use secure authentication methods like LOGIN or PLAIN over a TLS/SSL encrypted connection. Store user credentials securely, preferably not in plain text. Enabling TLS/SSL is non-negotiable for both incoming and outgoing connections whenever possible. It encrypts the communication channel, protecting credentials and message content from eavesdropping. Configure Exim to prefer or require TLS for connections supporting it.
Preventing abuse also involves rate limiting. Configure limits on the number of connections, messages, or recipients from a single IP address over a given time period. This helps mitigate the impact of denial-of-service attacks and spam botnets attempting to exploit your server. Implement checks against public DNS-based blackhole lists (DNSBLs), though use them judiciously and understand their potential false positives. Reject mail from IP addresses listed on reputable DNSBLs early in the SMTP conversation.
Furthermore, never run Exim as the root user. Configure it to drop privileges and run as a dedicated, unprivileged user (non-root user) as soon as possible after binding to the privileged SMTP port (25 or 587). This limits the damage if the Exim process is compromised.
Regular maintenance and updates are non-negotiable. Keep your Exim installation and the underlying operating system up-to-date with the latest security patches. Monitor your Exim logs diligently for suspicious activity, connection attempts from unusual locations, or excessive connection failures. Understanding the logs is essential for detecting and responding to security incidents.
By diligently configuring ACLs, enabling SMTP AUTH with TLS, implementing rate limiting, using DNSBLs, running as a non-root user, and performing regular updates, you can build a highly secure and reliable Exim mail server that stands up to modern threats. This level of rigorous configuration sets your server apart and ensures the integrity of your email operations.
Source: https://www.webhi.com/how-to/configure-secure-smtp-server-using-exim/
