Email server security best practices for MSPs and IT teams: 2025 definitive guide

Email remains a key entry point for cyberattacks. According to Verizon’s Data Breach Investigations Report, 94% of all malware is delivered via email. Phishing, spoofing, and business email compromise (BEC) attacks have also evolved with AI, rendering manual defenses ineffective. For managed service providers (MSPs) and IT teams, an unsecured email server directly endangers client security, regulatory compliance, and business operations.

In this blog, we’ll cover the essential email server security best practices MSPs and IT teams need to reduce risk in 2025.

Key takeaways

• Email is a top cyberattack vector: 94% of all malware is delivered through email, making email server security a critical priority for MSPs and IT teams.
• Authentication and encryption are non-negotiable: SPF, DKIM, DMARC, and TLS protect against spoofing, phishing, and data theft.
• Server hardening and configuration management reduce risks: Proper and timely security configurations close common entry points for attackers.
• User training reinforces technical defenses: Security awareness programs help prevent phishing and BEC attacks.
• Advanced monitoring and threat detection deliver stronger protection: Real-time monitoring and AI-driven tools improve visibility and incident response.

Why email security matters in 2025

Email attacks are no longer just spam campaigns; they are precise, AI-driven threats designed to bypass traditional defenses. Phishing, spoofing, and ransomware frequently start with an email, and the financial consequences are staggering: the average cost of a data breach in the United States is $10.22 million, according to IBM’s 2025 Cost of a Data Breach Report.

For MSPs and IT teams, the stakes are even higher:

• Reputation impact: A single breach can erode user trust.
• Compliance risk: Misconfigured servers jeopardize GDPR, HIPAA, and SOX compliance.
• Email deliverability: Poorly secured servers are flagged by spam filters, harming legitimate communication.

A secure email server protects not only sensitive information but also the operational integrity of every client network it touches.

How to secure your email server in 2025

An unsecured email server is one of the fastest ways for attackers to compromise your clients and your business. Traditional defenses such as spam filters and manual monitoring are no longer enough. MSPs and businesses need a layered security approach to significantly reduce risk.

The following email server security best practices give MSPs and IT teams a clear, actionable roadmap to strengthen defenses, minimize downtime, and ensure compliance.

1. Implement strong authentication and access controls

Authentication is the first line of defense against unauthorized access and account compromise. Without it, even the strongest filtering tools or monitoring systems leave critical gaps. Weak or reused passwords, combined with a lack of multi-factor authentication (MFA), create an easy target for attackers.

Best practices for authentication and access control:

• Enable MFA for all accounts: Require multi-factor authentication for administrators, service accounts, and users to block credential-stuffing attacks.
• Use SPF, DKIM, and DMARC: Enforce email authentication protocols to prevent spoofing and ensure email legitimacy.
• Apply role-based access control (RBAC): Grant the minimum permissions necessary to reduce the blast radius of any breach.
• Monitor login activity: Use logging and anomaly detection tools such as security information and event management (SIEM) to identify unusual login patterns or brute-force attempts.

The result: By enforcing strong authentication and limiting access, you create a hardened entry point that keeps attackers from exploiting stolen or weak credentials.

2. Encrypt email communications and data in transit

Once authentication is in place, encryption becomes the next critical layer of defense. Even with proper access controls, data is still at risk if it moves through the network unprotected. Encryption ensures that sensitive messages and attachments remain unreadable to unauthorized parties, even if intercepted during transit.

Best practices for email encryption:

• Enable transport layer security (TLS): Secure data in transit between email servers to prevent eavesdropping and tampering.
• Use end-to-end encryption for sensitive content: Implement tools that encrypt emails from sender to recipient, ensuring only intended parties can view the contents.
• Automate encryption policies: Configure rules to automatically encrypt messages containing sensitive data such as client credentials or financial information.
• Maintain encryption certificates: Regularly update and monitor certificates to avoid service disruptions or compliance gaps.

The result: Encryption adds a second line of defense, protecting confidential data and reinforcing trust with clients while supporting compliance with regulations such as HIPAA, GDPR, and SOC 2.

3. Harden your email server and continually monitor for vulnerabilities

Even with authentication and encryption in place, security vulnerabilities in the email server itself can create an entry point for attackers. Unconfigured or default security configurations and unnecessary services often become the weak links that threat actors exploit. Server hardening and vulnerability management close these gaps before they can be abused.

Best practices for email platform hardening:

• Disable unnecessary services: Reduce the attack surface by turning off unused or legacy services that could be exploited.
• Update security configurations promptly: As solution providers update security settings, MSPs and IT professionals should review and apply the configuration quickly to eliminate or reduce vulnerabilities.
• Enforce security settings: Use baseline configuration frameworks such as CIS Benchmarks to match platform security with best industry practices.
• Audit regularly: Conduct routine security assessments to identify misconfigurations and unauthorized changes.
• Leverage vulnerability scanning tools: Continuously monitor for and remediate potential server weaknesses with vulnerability management tools, such as ConnectWise Risk Assessment™.

The result: A hardened email platform minimizes exposure to known threats, making it significantly harder for attackers to gain a foothold in your environment.

4. Implement proactive monitoring and threat detection

Even a fully authenticated, encrypted, and hardened email server can still be targeted by evolving attack techniques. Proactive monitoring and threat detection provide real-time visibility into suspicious activity, allowing MSPs and IT teams to stop attacks before they escalate.

Best practices for monitoring and threat detection:

• Enable continuous logging: Collect and analyze email server logs to identify unusual activity, such as unauthorized access attempts or bulk email anomalies.
• Deploy AI-driven threat detection tools: Use security solutions that leverage machine learning to detect phishing, BEC, and other sophisticated attacks.
• Set up automated alerts: Configure alerts for key security events such as failed login attempts or configuration changes.
• Integrate with an XDR, MDR, or SIEM software: Centralize email security data for faster investigation and response across the environment. Alerts from email activity can be correlated with endpoint, network, and data protection signals, ensuring earlier detection of multi-vector ransomware or phishing campaigns.
• Review reports regularly: Conduct routine reviews of security dashboards and reports to validate that defenses are functioning properly.

The result: Proactive monitoring strengthens your email security posture by turning reactive defense into continuous prevention, reducing the risk of undetected breaches.

5. Implement spam and malware filtering

Email is one of the primary delivery methods for phishing, ransomware, and other malware attacks. Spam and malware filtering solutions act as the first line of defense by automatically identifying and blocking malicious messages before they reach end users. Adding an email firewall enhances this protection by applying advanced scanning, threat detection, and policy enforcement at the gateway.

Best practices for spam and malware filtering:

• Leverage AI-driven threat detection: Use machine learning to identify emerging email-borne threats that traditional filters might miss.
• Use advanced spam filters: Filter out phishing attempts, spoofed addresses, and unwanted bulk emails to reduce risk and improve email system performance.
• Apply outbound filtering: Prevent compromised accounts from being used to send spam or malicious emails, protecting your organization’s reputation.
• Regularly update filtering policies: Continuously refine rules to adapt to new phishing techniques and malware variants.

The result: Combining spam and malware filtering stops the majority of email-based attacks at the source, reduces user exposure to malicious content, and strengthens your organization’s overall email security posture.

6. Strengthen security awareness and user training

Even the most secure email server can be undermined if end users fall victim to phishing or social engineering attacks. MSPs and IT teams must pair technical safeguards with ongoing user education to reduce human error and strengthen overall defenses.

Best practices for security awareness and training:

• Run phishing simulations: Test user responses with realistic phishing campaigns to identify gaps and provide targeted coaching.
• Deliver regular training sessions: Offer short, recurring training to keep email security risks top-of-mind for employees.
• Teach safe email handling practices: Educate users on spotting suspicious links, verifying sender addresses, and reporting potential threats.
• Reinforce with policies: Document and enforce email security policies that define acceptable use and required security behaviors.
• Measure progress: Track training completion rates and phishing simulation results to demonstrate measurable improvement.

The result: A well-trained workforce acts as an additional layer of defense, complementing technical safeguards and reducing the likelihood of successful email-based attacks.

7. Still running an on-prem email server? Segmentation, isolation, and patching are key!

Network segmentation is a critical defense against lateral movement in your network. When email servers share the same environment with other critical systems, attackers who gain a foothold can pivot and compromise additional assets. Isolating your email infrastructure, along with timely patching, limit exposure. 

Best practices for segmentation and isolation:

• Deploy dedicated network zones: Place email servers in their own subnet or VLAN to separate them from core infrastructure and end user devices.
• Restrict inbound and outbound traffic: Use firewalls and access control lists (ACLs) to allow only necessary email-related traffic while blocking all unauthorized connections.
• Implement network-level monitoring: Continuously inspect traffic to and from the email server for anomalies or suspicious activity.
• Limit administrative access paths: Require secure VPNs or jump servers for administrative access to prevent direct exposure to the public internet.
• Keep servers fully patched and updated: Apply security updates promptly to close known vulnerabilities that attackers often exploit. Establish a regular patching schedule and test updates in a staging environment before production deployment.

The result: By segmenting, isolating, and patching on-prem email servers, you create a layered defense that stops attackers from moving laterally, containing any breach and protecting the rest of your environment.

8. Audit and log email activity

Visibility is essential for detecting suspicious activity and meeting compliance requirements. Without proper auditing and logging, malicious actions can go unnoticed, making it nearly impossible to investigate breaches or prove regulatory compliance. Centralized, well-structured logging enables MSPs and IT teams to identify anomalies early and respond before damage escalates.

Best practices for auditing and logging:

• Centralize log collection: Aggregate email server logs in a SIEM tool for real-time monitoring and analysis.
• Enable detailed audit trails: Track login attempts, configuration changes, and email flow activity to create a complete record for investigations.
• Set up automated alerts: Configure alerts for unusual login behavior, excessive failed login attempts, or suspicious email forwarding rules.
• Regularly review solution changes: Establish a routine review process to identify solution configuration or security changes to ensure adherence to security and compliance requirements.
• Retain logs securely: Store logs for a defined retention period based on industry standards or regulatory mandates.

The result: By auditing and logging email activity, you gain the visibility needed to detect threats, meet compliance requirements, and build a defensible security posture.

9. Test your defenses regularly

Even the most well-configured email security measures can weaken over time if they aren’t tested. Threat actors constantly evolve their techniques, and untested defenses create blind spots that attackers exploit. Regular testing validates your security posture, uncovers gaps, and ensures your email server protections align with current threat trends.

Best practices for testing email server defenses:

• Conduct regular penetration tests: Simulate real-world attacks to identify vulnerabilities in email authentication, filtering, and server configurations.
• Run phishing simulations: Test how users respond to phishing attempts and reinforce training for those who fall for simulated attacks.
• Perform vulnerability scans: Use automated scanning tools to detect misconfigurations or outdated software that could be exploited.
• Review security controls quarterly: Reassess access controls, authentication policies, and monitoring configurations for effectiveness.
• Document test results: Maintain clear reports to track improvements, demonstrate compliance, and prioritize remediation efforts.

The result: By testing email server defenses regularly, you reduce risk exposure, validate security investments, and maintain a proactive approach against evolving threats.

10. Establish email backup and disaster recovery

Even the strongest email security defenses cannot guarantee 100% prevention. Ransomware, accidental deletion, or server corruption can still disrupt business operations. A robust business continuity and disaster recovery strategy provides a safety net that minimizes downtime and data loss.

Best practices for email server backup and recovery:

• Automate email backups: Schedule frequent automated backups of mailboxes, configurations, and server data to eliminate gaps caused by manual processes.
• Use off-site or cloud-based storage: Store backups in secure, isolated environments to prevent them from being compromised during an attack.
• Test recovery procedures: Regularly perform test restores to validate that backups are intact and recovery time objectives (RTOs) can be met.
• Protect backups with encryption: Encrypt backup data in transit and at rest to keep sensitive information secure.
• Document recovery runbooks: Maintain clear, step-by-step recovery procedures so MSPs and IT teams can act quickly during an outage.

The result: A well-defined backup and disaster recovery strategy ensures email availability even in the event of an attack or critical failure, reducing downtime and safeguarding client trust.

While securing the email server is a critical first step, MSPs and IT teams must protect the entire email environment, including the underlying server and the email platform, especially in cloud-based systems such as Microsoft 365® and Google Workspace. Overlooking either layer can create exploitable gaps and significantly increase the risk of compromise.

Strengthen email security with ConnectWise and Proofpoint

Modern email attacks, from AI-driven phishing to ransomware, demand more than basic filtering. ConnectWise Email Security™ with Proofpoint gives MSPs enterprise-grade protection that includes email firewalls, URL and attachment sandboxing, automated threat removal, and centralized management within the ConnectWise Asio® platform. Backed by advanced AI and global threat intelligence, this solution blocks 99.99% of email-borne threats and is trusted by over 85% of Fortune 100 companies. Plus, it includes security awareness training (SAT), which is something every MSP needs.