SMTP Email Verifier

A Python script to check the deliverability of email addresses by:

1. Performing a quick syntax check.
2. Resolving the domain’s MX records (via dnspython).
3. Attempting an SMTP handshake to see if the mail server will accept the recipient.

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Table of Contents

1. Features
2. Prerequisites
3. Installation
4. Usage
5. How It Works
6. Caveats
7. FAQ
8. License

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Features

• Syntax Validation: Uses a simple regular expression to ensure the email follows standard local@domain.tld format.
• MX Record Lookup: Tries to locate the mail exchanger (MX) for the domain.
• SMTP Handshake: Connects to the mail server on port 25 and checks if it will accept the email address via MAIL FROM → RCPT TO commands.

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Prerequisites

• Python 3.6+ (Recommended: Python 3.10 or higher)
• pip or another package manager for Python.
• dnspython library for DNS lookups.

Note: Some ISPs or hosting environments block port 25 (SMTP). If that is the case, the script may fail to connect to mail servers, resulting in false INVALID outcomes.

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Installation

1. Clone or Download the Repository

   git clone https://github.com/mattlawlis/SMTP-Email-Verifier.git
   cd SMTP-Email-Verifier

2. Install Dependencies

   # Option 1: Install globally
   pip3 install dnspython
   
   # Option 2: Use a virtual environment
   python3 -m venv venv
   source venv/bin/activate
   pip install dnspython

3. Verify Installation

   pip3 show dnspython

   You should see dnspython’s version, location, and other info.

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Usage

1. Prepare emails.txt

   • In the same folder as smtp_email_verifier.py, create (or update) a file named emails.txt.
   • Put one email address per line, for example:

     example1@gmail.com
     user2@domain.org
     alice@unknown-domain.example
     

2. Run the Script

   • Open a terminal and navigate to the script’s directory:

     python3 smtp_email_verifier.py

   • If using a virtual environment:

     source venv/bin/activate
     python smtp_email_verifier.py

3. View the Results

   • The script prints each email address followed by VALID or INVALID.
   • A summary of valid vs. invalid emails is displayed at the end.

Example Output:

example1@gmail.com -> VALID
user2@domain.org -> INVALID
alice@unknown-domain.example -> INVALID

--- Summary ---
Total emails processed: 3
Valid: 1
Invalid: 2

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How It Works

1. Basic Syntax Check
   A simple regular expression validates the format local-part@domain.tld.

2. MX Record Lookup

   • The script extracts the domain (everything after @).
   • Uses dns.resolver.resolve(domain, 'MX') to fetch and sort MX records by priority.

3. SMTP Handshake

   • Attempts to connect to each MX server on port 25.
   • Sends:
     1. EHLO or HELO
     2. MAIL FROM:
     3. RCPT TO:
   • If the server returns a 250 code for RCPT TO, the email is considered VALID. Otherwise, it tries the next MX server or marks as INVALID.

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Caveats

1. Port 25 Blocking

   • Some networks block outbound SMTP on port 25, leading to connection failures.

2. Catch-All Domains

   • Some domains accept mail for any address. Such addresses might appear VALID even if they don’t truly exist.

3. Greylisting / Tarpitting

   • Some servers deliberately slow or block unknown senders, which may cause false negatives.

4. Rate Limiting / Blacklisting

   • Sending too many verification requests can trigger rate limits or blacklisting.

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FAQ

1. Why do I get ModuleNotFoundError: No module named 'dns'?
   Make sure dnspython is installed and that you’re not overshadowing the library with a local file named dns.py.

2. Why is a known good address marked INVALID?

   • The mail server may block verification attempts from unknown or suspicious IPs.
   • Temporary network issues could prevent connecting to the mail server.

3. Why is an address I know is invalid reported as VALID?

   • The domain might be a “catch-all” domain that accepts mail for any user.

4. Can I save the results to a file?

   • Yes. Modify the script to write the results to a CSV or text file, for example:

     with open("results.csv", "w") as out:
         out.write("Email,Status\n")
         for email in emails:
             status_msg = "VALID" if verify_smtp(email) else "INVALID"
             out.write(f"{email},{status_msg}\n")

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License

This project is licensed under the Apache License 2.0 - see the LICENSE file for details.

Use at your own risk, and feel free to modify or redistribute under the terms of the Apache 2.0 license.