Key Points
- Replace vendor default settings and passwords with complex, unique credentials to block automated attacks and brute force exploits.
- Require TLS for all email connections, secure SMTP submission with authentication, and use MTA-STS policies to strengthen encrypted communications between supporting mail servers.
- Configure SPF, DKIM, and DMARC to authenticate sending domains and reduce direct domain spoofing. Use matching forward and reverse DNS records to validate your sending infrastructure and support email deliverability.
- Restrict admin rights, limit access to authorized IPs or accounts, and enforce segmentation to minimize insider threats and unauthorized access.
- Keep mail servers patched, deploy firewalls/anti-spam filters to block malicious traffic, and regularly train employees on phishing and social engineering risks.
Email has become an indispensable tool for communication in both personal and professional spheres today. From exchanging critical business information to connecting with friends and family, email plays a pivotal role in our daily lives. However, with the increasing reliance on email comes the need to ensure its security, integrity, and reliability.
Email security is paramount to protecting sensitive data, maintaining message integrity, and thwarting potential threats such as phishing attacks, malware distribution, and unauthorized access. Organizations can mitigate risks and safeguard their email infrastructure against evolving cyber threats by implementing robust security measures. This guide explores email server security best practices and provides valuable insights for safeguarding email communications.
Prefer a visual walkthrough? Watch the video version of this guide: How to Use Email Server Security Best Practices.
What is a secure email server?
A secure email server is a system that sends, receives, or stores email while implementing security controls to protect email communications from unauthorized access or tampering. For a detailed guide on setting up an email server, you’re welcome to refer to NinjaOne’s comprehensive email server guide.
The role of a secure email server extends beyond mere message delivery. It is a cornerstone for data integrity and confidentiality, providing a secure channel for transmitting sensitive information, confidential documents, and critical business communications. These servers implement various encryption techniques, authentication mechanisms, access controls, and monitoring systems to safeguard sensitive information transmitted via email.
Key features of a secure email server
- Encryption: Secure email systems use Transport Layer Security (TLS) to encrypt email connections while messages are in transit. Storage encryption protects mailbox data at rest, while OpenPGP or S/MIME can provide message-level encryption when additional protection is required. These controls reduce the risk of email being intercepted or accessed by unauthorized parties.
- Authentication: Secure email systems use authentication mechanisms to verify the identities of users and systems accessing email services. This often involves multi-factor authentication (MFA) or digital certificates to prevent unauthorized access to email accounts.
- Access controls: Access controls are implemented to restrict access to email accounts and ensure that only authorized users can view, send, or modify emails. Role-based access controls (RBAC) and permission settings help enforce these access restrictions.
- Anti-malware and anti-spam protection: Secure email servers include built-in anti-malware and anti-spam filters to detect and block malicious attachments, phishing emails, and spam messages. These filters help prevent email-borne threats from reaching users’ inboxes.
- Data Loss Prevention (DLP): DLP mechanisms are employed to prevent the unauthorized disclosure of sensitive information through email. DLP policies can detect and block emails containing confidential data, such as Social Security numbers, credit card numbers, or intellectual property.
- Auditing and logging: Secure email servers maintain comprehensive audit logs of email activities, including message delivery, access attempts, and configuration changes. These logs are essential for monitoring and investigating security incidents or compliance violations.
- Secure configuration: Secure email servers are configured according to industry best practices and security standards to minimize the risk of vulnerabilities and ensure a hardened security posture. This includes regular patching, turning off unnecessary services, and implementing security baselines.
Use practical checks to strengthen systems, accounts, applications, and network configurations
10 email server security best practices
In addition to the technologies leveraged by secure email servers, the following configuration best practices should be adopted to optimize overall security posture:
#1. Change default passwords
Default passwords pose a significant security risk as attackers often exploit them. Changing default passwords promptly upon setting up an email server is essential to prevent unauthorized access. When creating new passwords, opt for long, unique ones that don’t appear on lists of commonly used or compromised credentials. Avoid easily guessed passwords such as “password123” or common phrases. Consider using a password manager to generate and securely store passwords, and require MFA for administrative accounts.
#2. Set up SMTP authentication
SMTP (Simple Mail Transfer Protocol) authentication is a mechanism used to verify the identity of users sending emails through an email server. Enabling SMTP authentication helps verify users, devices, and applications that submit ongoing messages through the server, minimizing the risk of spam and abuse. Configure the server to reject unauthorized relay attempts separately. For message submission, require TLS and authentication on the designated submission service, usually port 587 with STARTTLS or port 465 with implicit TLS. Use modern authentication or OAuth where supported.
#3. Protect with MTA STS
Mail Transfer Agent Strict Transport Security (MTA STS) is a security mechanism designed to enforce secure communication between mail servers, mitigating the risk of man-in-the-middle attacks and eavesdropping during email transmission. It helps supporting mail servers to require an encrypted connection and validate the receiving domain’s approved mail servers, strengthening email transport security. Implementing MTA-STS involves publishing a discovery TXT record in DNS and hosting the policy at the required HTTPS address. Your listed MX servers must also use valid TLS certificates. Supporting sending servers can then validate the approved MX hosts and enforce secure connections when delivering email to your domain. You can also configure TLS-RPT to receive reports about failed TLS connections and policy errors.
#4. Use secure email protocols
Transport Layer Security (TLS) encrypts email connections while messages are in transit, reducing the risk of interception and unauthorized access. It’s essential to configure your email server to use modern TLS for incoming and outgoing email connections. Ensure your email server software supports modern TLS for incoming and outgoing email connections. This involves enabling the appropriate encryption settings and ensuring that TLS certificates are properly installed and configured.
SSL and deprecated TLS versions should be disabled. Configure your server to use TLS 1.2 or later and prefer TLS 1.3 where supported.
#5. Configure reverse DNS
Reverse DNS (Domain Name System) associates an IP address with a hostname, helping receiving systems validate the infrastructure used to send email. Configuring reverse DNS for your email server supports sender reputation and email deliverability, but it does not authenticate the message’s visible From address. To set up reverse DNS, contact your DNS provider or hosting provider to create and configure reverse DNS records for your email server’s IP address. Ensure that the reverse DNS records accurately reflect the hostname and domain associated with your email server.
#6. Implement email firewalls
Email security solutions help detect and block spam, phishing attempts, malware, and other email-based threats before they reach users’ inboxes. These solutions can include secure email gateways, cloud-based email security services, and software-based email filtering. Choose a solution that aligns with your organization’s security requirements and integrates seamlessly with your email server.
#7. Update & patch servers regularly
Keeping email server software up to date is critical for addressing known vulnerabilities, bugs, and security flaws that attackers could exploit. Regular software updates and patches help maintain the integrity and security of your email server environment. Establish a routine schedule for applying updates and patches to your email server software, operating system, and associated components. Monitor vendor announcements, security advisories, and patch release notes to stay informed about the latest updates and security fixes.
#8. Configure SPF, DKIM, and DMARC to reduce spoofing
Sender Policy Framework (SPF) is an email authentication method that identifies which servers are authorized to send email for a domain. DKIM adds a cryptographic signature that receiving systems can verify, while DMARC checks whether the visible From domain aligns with a domain authenticated through SPF or DKIM. Using all three reduces direct domain spoofing and gives receiving systems clear instructions for messages that fail authentication. Access your domain’s DNS settings and add a TXT record containing the SPF policy information. Include every service authorized to send email for your domain. Enable DKIM signing through your email provider or server, then publish a DMARC record with aggregate reporting. Review the reports and correct legitimate authentication problems before moving from p=none to p=quarantine or p=reject.
#9. Limit access to your email server
Access control measures are essential for restricting unauthorized access to your email server and preventing potential security breaches. Implement granular access controls, user authentication mechanisms, and privilege management policies to control who can access sensitive email data and server resources. Define user roles, permissions, and access levels based on job responsibilities and the principle of least privilege. Utilize username/password authentication, multi-factor authentication (MFA), and IP-based access restrictions to authenticate and authorize users for secure server management.
#10. Provide training
Educating employees about email security best practices is crucial for building a security-aware culture and mitigating the risk of human error-related security incidents. Develop a comprehensive training program covering various aspects of email security, including identifying phishing emails, recognizing suspicious attachments, and practicing safe email usage habits. Conduct regular training sessions, workshops, and awareness campaigns to reinforce email security awareness among employees. Provide practical examples, real-world scenarios, and interactive learning materials to engage employees and promote active participation in email security training initiatives.
Evaluate centralized server monitoring, patching, and configuration management in your environment
Enhance the resilience and integrity of your email infrastructure
Email is integral to communications worldwide. However, with the convenience and ubiquity of email comes several security challenges, ranging from phishing attacks and malware distribution to data breaches and unauthorized access. Securing your email server is not just a matter of safeguarding sensitive information – it’s a fundamental aspect of maintaining business continuity, protecting customer trust, and upholding regulatory compliance standards.
