This document describes the security measures implemented in this project and recommendations for maintaining a secure environment when using msmtp on Ubuntu Server.

1. Defense in Depth

• Use AppArmor in enforce mode to confine msmtp.
• Strict TLS certificate validation (tls_certcheck on).

2. Credential Protection

• Use passwordeval to read passwords from files with 600 permissions and root:root.
• Store secrets in a /etc/msmtp/ directory with 700 permissions.
• Recommended: Use App Passwords instead of master passwords.

3. Secure Logs

• Logging to /var/log/msmtp/msmtp.log with restrictive permissions (640) and a controlled owner.
• A /var/log/msmtp/ folder with 750 permissions, compatible with AppArmor.
• Alternative: Use of syslog LOG_MAIL to centralize in /var/log/mail.log.

4. TLS and Encryption

• Use of secure ports: 587 (STARTTLS) or 465 (SMTPS).
• tls_trust_file pointing to the /etc/ssl/certs/ca-certificates.crt system bundle.
• Ability to harden ciphers with tls_priorities.

5. Permissions and Isolation

• Avoid sensitive logs or files in $HOME in multi-user environments.
• One password file per SMTP account, with minimal permissions.
• Separate system accounts for services that send email.

• Saving passwords in plain text in msmtprc.
• Disabling tls_certcheck in production.
• Using lax permissions (777) on directories or configuration files.
• Disabling AppArmor without a solid technical justification.

• Official msmtp manual
• AppArmor documentation on Ubuntu