Email security is critical for protecting both your organization and your recipients. Email systems are frequent targets for attackers, and a compromised email infrastructure can damage your reputation, expose sensitive data, and enable attacks against your users.
This guide covers the security considerations for email sending infrastructure and the practices that protect against common threats.
Threat Landscape
Credential Theft
Attackers target email credentials to send spam through your infrastructure, access sensitive communications, impersonate your organization, and damage your sender reputation.
API Key Exposure
Exposed API keys allow unauthorized sending from your account, access to recipient data, potential billing fraud, and reputation damage from spam.
Infrastructure Compromise
Compromised email infrastructure enables mass spam campaigns, phishing attacks using your domain, data exfiltration, and service disruption.
Data Exposure
Email content and metadata can expose personal information, business secrets, authentication tokens, and internal communications.
Authentication Security
API Key Management
Generation
Use cryptographically secure random generation for API keys. Generate sufficiently long keys with at least 256 bits of entropy. Use different keys for different purposes and environments.
Storage
Never store API keys in source code. Use environment variables or secrets management systems. Encrypt keys at rest. Limit access to key storage.
Rotation
Rotate keys periodically as a best practice. Rotate immediately if compromise is suspected. Support multiple active keys for zero-downtime rotation. Revoke old keys after transition period.
Scoping
Apply least privilege principle. Create keys with only needed permissions. Use separate keys for different services. Limit IP addresses that can use each key.
SMTP Authentication
Always require authentication for SMTP submission. Use TLS encryption for all SMTP connections. Implement rate limiting per authenticated user. Monitor for unusual authentication patterns.
Transport Security
TLS for All Connections
Require TLS for all API connections. Use TLS for SMTP submission. Configure MTA to prefer TLS for delivery. Use current TLS versions (1.2 or higher).
Certificate Management
Use certificates from trusted CAs. Monitor certificate expiration. Implement automatic renewal. Validate certificates in client connections.
DANE and MTA-STS
DANE (DNS-based Authentication of Named Entities) ties TLS certificates to DNS. MTA-STS (Mail Transfer Agent Strict Transport Security) signals TLS requirements for receiving. Both improve transport security for email delivery.
Content Security
Sensitive Data Handling
Avoid sending highly sensitive data in email when possible. Do not include passwords, full credit card numbers, or similar sensitive information. Use secure links to view sensitive content. Implement data masking for logs.
Attachment Security
Scan attachments for malware. Limit attachment types and sizes. Consider hosting files and sending links instead. Implement content inspection for outbound email.
Link Security
Validate URLs before including in emails. Use HTTPS for all links. Consider link wrapping for click tracking. Monitor for malicious link insertion.
Abuse Prevention
Rate Limiting
Implement rate limits per API key, per sending domain, per recipient domain, and per time period. This prevents abuse if credentials are compromised and limits damage from bugs.
Content Filtering
Scan outbound email for spam characteristics, malicious URLs, phishing indicators, and policy violations. Block or flag suspicious content before sending.
Recipient Validation
Validate email addresses at collection. Implement double opt-in. Check against known bad actors. Monitor for list bombing attacks.
Volume Monitoring
Alert on unusual volume spikes. Monitor sending patterns. Implement automatic throttling. Require approval for volume increases.
Infrastructure Security
Network Security
Firewall email infrastructure appropriately. Isolate email systems from other services. Use private networks for internal communication. Implement intrusion detection.
Server Hardening
Keep systems patched and updated. Disable unnecessary services. Use strong authentication for server access. Implement access logging and monitoring.
Monitoring and Alerting
Monitor for authentication failures, unusual sending patterns, reputation changes, and security events. Alert security team on suspicious activity.
Incident Response
Preparation
Have an incident response plan for email security events. Define roles and responsibilities. Document escalation procedures. Practice response scenarios.
Detection
Monitoring should catch abuse quickly. Monitor bounce and complaint rates. Watch for reputation drops. Check blocklist status regularly.
Containment
Be prepared to quickly revoke compromised credentials, block suspicious sending, isolate affected systems, and preserve evidence for investigation.
Recovery
Identify root cause. Remediate vulnerabilities. Rotate credentials. Rebuild reputation if damaged. Document lessons learned.
Compliance and Privacy
Data Protection
Encrypt data at rest. Minimize data retention. Implement access controls. Audit data access. Comply with applicable regulations.
Logging and Auditing
Log security-relevant events. Retain logs appropriately. Protect log integrity. Enable audit trails for sensitive operations.
Third-Party Security
Vet email service providers for security practices. Review data handling agreements. Ensure compliance with your requirements. Monitor for vendor security incidents.
Security Checklist
Authentication
- API keys stored securely, not in code
- Key rotation process defined
- Least privilege applied to credentials
- Multi-factor authentication for account access
Transport
- TLS required for all connections
- Current TLS versions only
- Certificates valid and monitored
Abuse Prevention
- Rate limiting implemented
- Content filtering active
- Monitoring for unusual patterns
- Incident response plan ready
Infrastructure
- Systems patched and hardened
- Network segmentation in place
- Access logging enabled
- Regular security assessments
Conclusion
Email security requires attention at multiple layers - authentication, transport, content, infrastructure, and operations. A compromised email system affects not just your organization but everyone you communicate with.
Invest in security proactively. The cost of prevention is far less than the cost of a breach.
GetMailer provides enterprise-grade security including TLS encryption, API key management, abuse prevention, and compliance features. Our infrastructure is designed with security as a fundamental requirement, not an afterthought.