Let’s explore SMTP Enumeration—a technique used in cybersecurity to gather information about email servers and user accounts 📧🔍.

📬 What Is SMTP?

Simple Mail Transfer Protocol (SMTP) is the protocol used to send emails across the Internet. It typically runs on port 25, 587, or 465.

🕵️‍♂️ What Is SMTP Enumeration?

SMTP Enumeration is the process of interacting with an SMTP server to:

• Discover valid email addresses or usernames
• Identify the mail server software and version
• Check for misconfigurations or vulnerabilities

This is often part of reconnaissance in penetration testing or ethical hacking.

🎯 Goals of SMTP Enumeration

• Validate user accounts (e.g., admin@example.com)
• Identify potential targets for phishing or spoofing
• Detect open relays or weak authentication

🔧 SMTP Commands Used in Enumeration

⚠️ Note: Many modern SMTP servers disable VRFY and EXPN for security reasons.

🛠️ Tools for SMTP Enumeration

1. Telnet (Manual Testing)

telnet mail.example.com 25
HELO example.com
VRFY admin

2. Nmap Scripts

nmap -p 25 --script smtp-enum-users <target-ip>

3. smtp-user-enum

A dedicated tool for brute-forcing SMTP usernames.

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

4. Metasploit

Metasploit has modules for SMTP enumeration and exploitation.

🔍 What You Can Discover

• Valid email addresses (e.g., john.doe@example.com)
• Mail server software (e.g., Postfix, Exchange)
• Open relays (servers that send mail without authentication)
• Misconfigured authentication mechanisms

⚠️ Security Implications

SMTP enumeration can lead to:

• Phishing attacks (targeting valid users)
• Email spoofing (if SPF/DKIM/DMARC are misconfigured)
• Spam relay abuse (if open relay is detected)
• Credential brute-forcing (if authentication is weak)