Conduct comprehensive security assessments of SMTP (Simple Mail Transfer Protocol) servers to identify vulnerabilities including open relays, user enumeration, weak authentication, and misconfiguration.

⚠️ AUTHORIZED USE ONLY This skill is for educational purposes or authorized security assessments only. You must have explicit, written permission from the system owner before using this tool. Misuse of this tool is illegal and strictly prohibited. Mandatory confirmation gate Before running any command that probes, exploits, changes, persists on, extracts data from, or attempts credential access against a target: 1. Ask the user to state the exact target URL, IP, account, or resource. 2. Ask the user to confirm written authorization and the permitted scope. 3. Show the exact command(s) and explain their expected effect. 4. Wait for explicit confirmation in the current conversation. Without that confirmation, remain read only and provide defensive guidance only. Prefer a sandbox, disposable VM, or controlled lab. AUTHORIZED USE ONLY: Use this skill only for authorized security assessments, defensive validation, or controlled educational environments. SMTP Penetration Testing Detailed Guide Read [the detailed guide](references/detailed guide.md) before executing this skill. It retains the complete procedure and reference material. Treat its safety, prerequisites, and validation requirements as mandatory. For focused work, load the relevant sections; for end to end work, read the guide completely. Prerequisites Required Tools Required Knowledge SMTP protocol fundamentals Email architecture (MTA, MDA, MUA) DNS and MX records Network protocols Required Access Target SMTP server IP/hostname Written authorization for testing Wordlists for enumeration and brute force Constraints and Limitations Legal Requirements Only test SMTP servers you own or have authorization to test Sending spam or malicious emails is illegal Document all testing activities Do not abuse discovered open relays Technical Limitations VRFY/EXPN often disabled on modern servers Rate limiting may slow enumeration Some servers respond identically for valid/invalid users Greylisting may delay enumeration responses Ethical Boundaries Never send actual spam through discovered relays Do not harvest email addresses for malicious use Report open relays to server administrators Use findings only for authorized security improvement Examples Example 1: Complete SMTP Assessment Scenario: Full security assessment of mail server Example 2: User Enumeration Attack Scenario: Enumerate valid users for phishing preparation Example 3: Open Relay Exploitation Scenario: Test and document open relay vulnerability Security Recommendations For Administrators 1. Disable Open Relay Require authentication for external delivery 2. Disable VRFY/EXPN Prevent user enumeration 3. Enforce TLS Require STARTTLS for all connections 4. Implement SPF/DKIM/DMARC Prevent email spoofing 5. Rate Limiting Prevent brute force attacks 6. Account Lockout Lock accounts after failed attempts 7. Banner Hardening Minimize server information disclosure 8. Log Monitoring Alert on suspicious activity 9. Patch Management Keep SMTP software updated 10. Access Controls Restrict SMTP to authorized IPs When to Use This skill is applicable to execute the workflow or actions described in the overview.

smtp-penetration-testing