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

# 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:

# 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:

# 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:

# 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:

# VRFY - Verify user exists
VRFY admin
250 2.1.5 [email protected]

# EXPN - Expand mailing list
EXPN staff
250 2.1.5 [email protected]
250 2.1.5 [email protected]

# RCPT TO - Recipient verification
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# 250 OK = user exists
# 550 = user doesn't exist

Phase 5: User Enumeration

Enumerate valid email addresses:

# 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:

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:

# 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:

# Using Nmap
nmap -p 25 --script smtp-open-relay TARGET_IP

# Manual testing via Telnet
telnet TARGET_IP 25
HELO attacker.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Relay Test
This is a test.
.
QUIT

# If accepted (250 OK), server is open relay

Using Metasploit:

use auxiliary/scanner/smtp/smtp_relay
set RHOSTS TARGET_IP
run

Test variations:

# Test different sender/recipient combinations
MAIL FROM:<>
MAIL FROM:<test@[attacker_IP]>
MAIL FROM:<[email protected]>

RCPT TO:<[email protected]>
RCPT TO:<"[email protected]">
RCPT TO:<test%[email protected]>

Phase 7: Brute Force Authentication

Test for weak SMTP credentials:

# 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.txt -P passwords.txt TARGET_IP smtp

# Verbose output
hydra -l admin -P passwords.txt smtp://TARGET_IP -V

Using Medusa:

medusa -h TARGET_IP -u admin -P /path/to/passwords.txt -M smtp

Using Metasploit:

use auxiliary/scanner/smtp/smtp_login
set RHOSTS TARGET_IP
set USER_FILE /path/to/users.txt
set PASS_FILE /path/to/passwords.txt
set VERBOSE true
run

Phase 8: SMTP Command Injection

Test for command injection vulnerabilities:

# Header injection test
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Test
Bcc: [email protected]
X-Injected: malicious-header

Injected content
.

Email spoofing test:

# Spoofed sender (tests SPF/DKIM protection)
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
From: CEO <[email protected]>
Subject: Urgent Request
Please process this request immediately.
.

Phase 9: TLS/SSL Security Testing

Test encryption configuration:

# STARTTLS support check
openssl s_client -connect TARGET_IP:25 -starttls smtp

# Direct SSL (port 465)
openssl s_client -connect TARGET_IP:465

# Cipher enumeration
nmap --script ssl-enum-ciphers -p 25 TARGET_IP

Phase 10: SPF, DKIM, DMARC Analysis

Check email authentication records:

# SPF/DKIM/DMARC record lookups
dig TXT target.com | grep spf            # SPF
dig TXT selector._domainkey.target.com    # DKIM
dig TXT _dmarc.target.com                 # DMARC

# SPF policy: -all = strict fail, ~all = soft fail, ?all = neutral

Quick Reference

Essential SMTP Commands

Command	Purpose	Example
HELO	Identify client	HELO client.com
EHLO	Extended HELO	EHLO client.com
MAIL FROM	Set sender	MAIL FROM:<[email protected]>
RCPT TO	Set recipient	RCPT TO:<[email protected]>
DATA	Start message body	DATA
VRFY	Verify user	VRFY admin
EXPN	Expand alias	EXPN staff
QUIT	End session	QUIT

SMTP Response Codes

Code	Meaning
220	Service ready
221	Closing connection
250	OK / Requested action completed
354	Start mail input
421	Service not available
450	Mailbox unavailable
550	User unknown / Mailbox not found
553	Mailbox name not allowed

Enumeration Tool Commands

Tool	Command
smtp-user-enum	smtp-user-enum -M VRFY -U users.txt -t IP
Nmap	nmap --script smtp-enum-users -p 25 IP
Metasploit	use auxiliary/scanner/smtp/smtp_enum
Netcat	nc IP 25 then manual commands

Common Vulnerabilities

Vulnerability	Risk	Test Method
Open Relay	High	Relay test with external recipient
User Enumeration	Medium	VRFY/EXPN/RCPT commands
Banner List & Monetize Your Skill Submit your Claude Code skill and start earning Get started → Use Cases Task Automation & Efficiency Automate repetitive workflows and reduce manual effort Example Generate reports, summarize documents, draft communications ✓ Save 3-5 hours per week on routine tasks Knowledge Enhancement Learn new skills, understand complex topics, get expert guidance Example Explain concepts, provide examples, suggest learning resources ✓ Accelerate learning and skill development by 2x Quality Improvement Enhance output quality through reviews, suggestions, and refinements Example Review drafts, suggest improvements, catch errors ✓ Improve work quality by 30-40% with less effort Implementation Guide Prerequisites ›Claude Desktop or compatible AI client with skill support ›Clear understanding of task or problem to solve ›Willingness to iterate and refine outputs Time Estimate 15-45 minutes depending on use case complexity Steps 1Install skill using provided installation command 2Test with simple use case relevant to your work 3Evaluate output quality and relevance 4Iterate on prompts to improve results 5Integrate into regular workflow if valuable Common Pitfalls ⚠Expecting perfect results without iteration ⚠Not providing enough context in prompts ⚠Using skill for tasks outside its intended scope ⚠Accepting outputs without review and validation Best Practices ✓ Do +Start with clear, specific prompts +Provide relevant context and constraints +Review and refine all outputs before using +Iterate to improve output quality +Document successful prompt patterns ✗ Don't −Don't use without understanding skill limitations −Don't skip validation of outputs −Don't share sensitive information in prompts −Don't expect skill to replace human judgment 💡 Pro Tips ★Be specific about desired format and style ★Ask for multiple options to choose from ★Request explanations to understand reasoning ★Combine AI efficiency with human expertise When to Use This ✓ Use when Use when skill capabilities match your task, clear ROI on time saved, and you can validate outputs. Best for repetitive tasks, learning, and quality improvement. ✗ Avoid when Avoid when task requires deep expertise you can't validate, involves sensitive decisions, or when learning process is more valuable than speed of completion. Learning Path 1Familiarize yourself with skill capabilities and limitations 2Start with low-risk, non-critical tasks 3Progress to more complex and valuable use cases 4Build expertise through regular use and experimentation Related Skills ml-paper-writing 66 davila7/claude-code-templates AI/MLsame repo docker-expert 12 davila7/claude-code-templates Cloudsame repo remotion-best-practices 10 davila7/claude-code-templates Videosame repo senior-data-engineer 8 davila7/claude-code-templates Productivitysame repo telegram-mini-app 8 davila7/claude-code-templates Productivitysame repo senior-backend 7 davila7/claude-code-templates Backendsame repo Reviews 4.5★★★★★73 reviews M Min Reddy★★★★★Dec 20, 2024 I recommend smtp-penetration-testing for anyone iterating fast on agent tooling; clear intent and a small, reviewable surface area. G Ganesh Mohane★★★★★Dec 16, 2024 I recommend smtp-penetration-testing for anyone iterating fast on agent tooling; clear intent and a small, reviewable surface area. K Kabir Khan★★★★★Dec 16, 2024 smtp-penetration-testing reduced setup friction for our internal harness; good balance of opinion and flexibility. N Noah Abebe★★★★★Dec 16, 2024 Keeps context tight: smtp-penetration-testing is the kind of skill you can hand to a new teammate without a long onboarding doc. K Kiara Shah★★★★★Dec 16, 2024 Solid pick for teams standardizing on skills: smtp-penetration-testing is focused, and the summary matches what you get after install. M Min Thomas★★★★★Dec 16, 2024 smtp-penetration-testing fits our agent workflows well — practical, well scoped, and easy to wire into existing repos. S Sophia Singh★★★★★Nov 19, 2024 Useful defaults in smtp-penetration-testing — fewer surprises than typical one-off scripts, and it plays nicely with `npx skills` flows. Y Yash Thakker★★★★★Nov 15, 2024 Useful defaults in smtp-penetration-testing — fewer surprises than typical one-off scripts, and it plays nicely with `npx skills` flows. O Omar Bansal★★★★★Nov 11, 2024 Registry listing for smtp-penetration-testing matched our evaluation — installs cleanly and behaves as described in the markdown. K Kofi Martinez★★★★★Nov 11, 2024 Solid pick for teams standardizing on skills: smtp-penetration-testing is focused, and the summary matches what you get after install. showing 1-10 of 73 prev1 / 8next Discussion Comments — not star reviews Post comment No comments yet — start the thread. sharebookmark