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SMTP-User-Enum

SMTP-User-Enum is a tool for enumerating valid usernames on SMTP servers using various techniques including VRFY, EXPN, and RCPT TO commands.

Installation

Linux/Ubuntu

# Install from repository
sudo apt update
sudo apt install smtp-user-enum

# Or from source
wget http://pentestmonkey.net/tools/smtp-user-enum/smtp-user-enum-1.2.tar.gz
tar -xzf smtp-user-enum-1.2.tar.gz
cd smtp-user-enum-1.2
chmod +x smtp-user-enum.pl

Kali Linux

# Pre-installed
smtp-user-enum -h

# Or install
sudo apt install smtp-user-enum

macOS

# Via Homebrew
brew install smtp-user-enum

# Or from source
git clone https://github.com/smtptools/smtp-user-enum.git
cd smtp-user-enum
perl smtp-user-enum.pl -h

Basic Usage

# Show help
smtp-user-enum -h

# VRFY method
smtp-user-enum -M VRFY -U wordlist.txt -t mail.example.com

# EXPN method
smtp-user-enum -M EXPN -U wordlist.txt -t mail.example.com

# RCPT TO method (default)
smtp-user-enum -M RCPT -U wordlist.txt -t mail.example.com

# All methods
smtp-user-enum -M ALL -U wordlist.txt -t mail.example.com

SMTP Enumeration Methods

VRFY (Verify Command)

# Basic VRFY enumeration
smtp-user-enum -M VRFY -U users.txt -t mail.example.com

# With custom port
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -p 25

# Verbose output
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -v

# Save results
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -o vrfy_results.txt

EXPN (Expand Command)

# EXPN enumeration
smtp-user-enum -M EXPN -U users.txt -t mail.example.com

# With timeout
smtp-user-enum -M EXPN -U users.txt -t mail.example.com -w 5

# Group enumeration
smtp-user-enum -M EXPN -U groups.txt -t mail.example.com

# Verbose with output
smtp-user-enum -M EXPN -U users.txt -t mail.example.com -v -o expn_results.txt

RCPT TO (Recipient Validation)

# RCPT TO enumeration (default)
smtp-user-enum -U users.txt -t mail.example.com

# Manual RCPT method
smtp-user-enum -M RCPT -U users.txt -t mail.example.com

# Fast RCPT enumeration
smtp-user-enum -M RCPT -U users.txt -t mail.example.com -t 2

# Different domain
smtp-user-enum -M RCPT -U users.txt -t mail.example.com -D example.org

Wordlist Management

Creating Wordlists

# Common user list
cat > users.txt << EOF
admin
root
postmaster
info
support
sales
test
user
EOF

# From file
cat /usr/share/wordlists/seclists/Usernames/top-usernames-shortlist.txt > users.txt

# Generate from CSV
awk -F',' '{print $1}' employees.csv > users.txt

Comprehensive Enumeration

# User enumeration from multiple sources
cat users_admin.txt users_it.txt users_generic.txt > combined_users.txt

# Remove duplicates
sort -u combined_users.txt > unique_users.txt

# Large wordlist (be cautious)
smtp-user-enum -M VRFY -U /usr/share/wordlists/common-users.txt -t mail.example.com

Advanced Options

Timeout and Throttling

# Custom timeout (seconds)
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -w 10

# Connection timeout
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -s 5

# Delay between requests (throttle)
# Note: Not all versions support this directly

Domain Specification

# Default domain
smtp-user-enum -M RCPT -U users.txt -t mail.example.com -D example.com

# Different SMTP domain
smtp-user-enum -M RCPT -U users.txt -t mail.example.com -D mail.example.org

# Test multiple domains
for domain in example.com example.org example.net; do
    smtp-user-enum -M RCPT -U users.txt -t mail.example.com -D $domain
done

Output Options

# Save results to file
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -o results.txt

# Verbose output
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -v

# Very verbose (debug)
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -vv

# Quiet mode
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -q

Manual SMTP Enumeration

Netcat Method

# Connect to SMTP server
nc -v mail.example.com 25

# Commands (interact manually)
VRFY admin
EXPN support
RCPT TO:<user@example.com>
QUIT

Using Telnet

# Connect
telnet mail.example.com 25

# Test commands
VRFY postmaster
EXPN admin
RCPT TO:<test@example.com>

Python Script Method

#!/usr/bin/env python3
import socket

def check_user(server, port, user, method='VRFY'):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((server, port))
    banner = s.recv(1024)
    print(f"[*] {banner.decode()}")

    # Send command
    cmd = f"{method} {user}\r\n"
    s.send(cmd.encode())
    response = s.recv(1024)

    s.close()
    return response.decode()

# Test
server = "mail.example.com"
port = 25
users = ["admin", "postmaster", "root"]

for user in users:
    print(f"[*] Testing {user}...")
    result = check_user(server, port, user)
    print(f"[+] {result}")

Complete Enumeration Workflow

#!/bin/bash
# Comprehensive SMTP enumeration script

TARGET="mail.example.com"
WORDLIST="/usr/share/wordlists/common-users.txt"
OUTPUT_DIR="smtp_enum_$(date +%Y%m%d_%H%M%S)"

mkdir -p $OUTPUT_DIR

echo "[*] Starting SMTP enumeration against $TARGET"

# Test connectivity
echo "[*] Testing SMTP connectivity..."
timeout 5 bash -c "echo > /dev/tcp/$TARGET/25" && echo "[+] SMTP port open" || echo "[-] SMTP port closed"

# VRFY enumeration
echo "[*] Running VRFY enumeration..."
smtp-user-enum -M VRFY -U $WORDLIST -t $TARGET -o $OUTPUT_DIR/vrfy_results.txt -v

# EXPN enumeration
echo "[*] Running EXPN enumeration..."
smtp-user-enum -M EXPN -U $WORDLIST -t $TARGET -o $OUTPUT_DIR/expn_results.txt -v

# RCPT enumeration
echo "[*] Running RCPT enumeration..."
smtp-user-enum -M RCPT -U $WORDLIST -t $TARGET -D example.com -o $OUTPUT_DIR/rcpt_results.txt -v

# Combine results
echo "[*] Combining results..."
cat $OUTPUT_DIR/*.txt | grep -i "^250\|^252" | cut -d: -f1 | sort -u > $OUTPUT_DIR/valid_users.txt

echo "[+] Enumeration complete!"
echo "[*] Valid users found: $(wc -l < $OUTPUT_DIR/valid_users.txt)"
cat $OUTPUT_DIR/valid_users.txt

Integration with Other Tools

Combine with Metasploit

# Enumerate users
smtp-user-enum -M VRFY -U users.txt -t mail.example.com -o found_users.txt

# Use in Metasploit
# scanner/smtp/smtp_enum
# set RHOSTS mail.example.com
# set USER_FILE found_users.txt

Password Spraying

# After finding valid users
cat valid_users.txt | while read user; do
    # Attempt auth with common password
    sendmail_auth.py $user password mail.example.com
done

Detecting Detection

Identifying Filters

# Test server response patterns
echo "Testing spam filter..."
smtp-user-enum -M VRFY -U test_invalid.txt -t mail.example.com

# If all return success, likely filtering VRFY
# Test with small list first
smtp-user-enum -M VRFY -U top_users.txt -t mail.example.com

Response Analysis

# Check response codes
# 250 = User exists (Positive)
# 550/551/552 = User doesn't exist (Negative)
# 421 = Service unavailable (Timeout/Blocked)

smtp-user-enum -M VRFY -U users.txt -t mail.example.com -v | grep "^5[0-9][0-9]"

Security Considerations

Defense Against Enumeration

  • Disable VRFY and EXPN commands
  • Uniform responses for valid/invalid users
  • Rate limiting on RCPT attempts
  • SMTP AUTH required
  • Monitor enumeration attempts

Responsible Testing

  • Only test authorized targets
  • Use approved wordlists
  • Don’t spam or overload servers
  • Document findings properly
  • Report responsibly

Common Response Codes

CodeMeaning
250Command OK
252Cannot verify (user may exist)
421Service unavailable
500Command unrecognized
550User not found
551User not local
552Storage exceeded
553Mailbox name invalid
  • Metasploit - smtp_enum module
  • Nessus - SMTP enumeration scan
  • Nmap - smtp-enum NSE script
  • Hydra - SMTP password brute-force

Last updated: 2026-03-30 | SMTP-User-Enum v1.2