EML RFC 822 mail

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🏷️ .eml

🎯 message/rfc822

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File Information

File Description

RFC 822 mail

EML (Email Message) File Format

Overview

The EML format is a standard file format for storing individual email messages. Based on the Internet Message Format standard (RFC 5322, formerly RFC 822), EML files contain the complete email message including headers, body text, and attachments in a plain text format. This format is widely used by email clients for message storage and exchange.

Technical Details

File Characteristics

Extension: .eml
MIME Type: message/rfc822
Category: Email
Format Type: Plain text with structured headers

Standard Compliance

RFC 5322: Internet Message Format
RFC 2045-2049: MIME (Multipurpose Internet Mail Extensions)
RFC 2822: Previous Internet Message Format standard
RFC 6854: Updated message format specification

File Structure

Basic Message Format

EML File Structure:
├── Header Fields
│   ├── From: [email protected]
│   ├── To: [email protected]
│   ├── Subject: Message Subject
│   ├── Date: Wed, 01 Jan 2024 12:00:00 +0000
│   └── Message-ID: <[email protected]>
├── Empty Line (separator)
└── Message Body
    ├── Plain Text Content
    └── MIME Parts (if multipart)

Complete EML Example

From: John Doe <[email protected]>
To: Jane Smith <[email protected]>
Subject: Project Meeting Tomorrow
Date: Wed, 15 Mar 2024 14:30:00 -0500
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

Hi Jane,

Don't forget about our project meeting tomorrow at 2 PM.
We'll be discussing the Q2 deliverables.

Best regards,
John

Header Fields

Standard Headers

Essential Headers:
From:        # Sender identification
To:          # Primary recipients
Cc:          # Carbon copy recipients
Bcc:         # Blind carbon copy recipients
Subject:     # Message subject line
Date:        # Message creation timestamp
Message-ID:  # Unique message identifier

Extended Headers

Additional Headers:
Reply-To:         # Reply address
Return-Path:      # Bounce address
Received:         # Mail server path
X-Mailer:         # Email client identification
X-Priority:       # Message priority
Organization:     # Sender organization
User-Agent:       # Email client software

Custom Headers

Custom Headers (X- prefix):
X-Spam-Score:     # Spam filtering score
X-Original-To:    # Original recipient
X-Forwarded-For:  # Forwarding information
X-Company-Info:   # Organization-specific data

MIME Support

Multipart Messages

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary123"

--boundary123
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

This is the plain text part of the message.

--boundary123
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<html>
<body>
<p>This is the <strong>HTML</strong> part of the message.</p>
</body>
</html>

--boundary123--

Content Types

Common MIME Types:
text/plain          # Plain text content
text/html           # HTML formatted content
multipart/mixed     # Mixed content types
multipart/alternative # Alternative representations
multipart/related   # Related parts (HTML + images)
application/pdf     # PDF attachment
image/jpeg          # JPEG image attachment

Encoding Methods

Transfer Encodings:
7bit               # 7-bit ASCII (default)
8bit               # 8-bit characters
quoted-printable   # Readable encoding for text
base64             # Binary data encoding
binary             # Raw binary data

Attachments

File Attachment Example

Content-Type: application/pdf; name="document.pdf"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="document.pdf"

JVBERi0xLjcKCjEgMCBvYmoKPDwKL1R5cGUgL0NhdGFsb2cKL1BhZ2VzIDIgMCBSCj4+
CmVuZG9iagoKMiAwIG9iago8PAovVHlwZSAvUGFnZXMKL0tpZHMgWzMgMCBSXQovQ291
bnQgMQo+PgplbmRvYmoK

Inline Images

Content-Type: image/png; name="logo.png"
Content-Transfer-Encoding: base64
Content-ID: <[email protected]>
Content-Disposition: inline; filename="logo.png"

iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg==

Email Client Support

Desktop Clients

EML Support:
- Microsoft Outlook (Windows)
- Mozilla Thunderbird (Cross-platform)
- Apple Mail (macOS)
- Windows Mail (Windows 10/11)
- Evolution (Linux)
- Claws Mail (Cross-platform)

Web-based Access

Most webmail clients can import EML files
Browser extensions for EML viewing
Online EML viewers and converters
Email archiving services

Programming Interface

Python Email Processing

import email
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.base import MIMEBase
from email import encoders

# Read EML file
with open('message.eml', 'r') as f:
    msg = email.message_from_file(f)

# Extract headers
sender = msg['From']
subject = msg['Subject']
date = msg['Date']

# Extract body
if msg.is_multipart():
    for part in msg.walk():
        if part.get_content_type() == "text/plain":
            body = part.get_payload(decode=True).decode()
else:
    body = msg.get_payload(decode=True).decode()

# Create new EML
msg = MIMEMultipart()
msg['From'] = '[email protected]'
msg['To'] = '[email protected]'
msg['Subject'] = 'Test Message'

# Add body
msg.attach(MIMEText('Message body', 'plain'))

# Save to file
with open('new_message.eml', 'w') as f:
    f.write(msg.as_string())

JavaScript Email Parsing

// Using emailjs-mime-parser
import { parse } from 'emailjs-mime-parser';

// Read EML file content
fetch('message.eml')
  .then(response => response.text())
  .then(emlContent => {
    const parsed = parse(emlContent);
    
    console.log('From:', parsed.headers.from[0].address);
    console.log('Subject:', parsed.headers.subject[0].value);
    console.log('Date:', parsed.headers.date[0].value);
    
    // Extract text content
    const textPart = parsed.childNodes.find(
      node => node.contentType.value === 'text/plain'
    );
    if (textPart) {
      console.log('Body:', textPart.content);
    }
  });

Common Use Cases

Email Archiving

Legal Compliance: Long-term message storage
Backup Solutions: Email backup and recovery
Migration: Moving between email systems
Forensics: Email investigation and analysis

Development and Testing

Email Templates: Storing template messages
Testing: Email functionality testing
Debugging: Email delivery troubleshooting
Integration: System-to-system email exchange

Data Exchange

Automated Systems: Machine-generated emails
Reporting: Automated report distribution
Notifications: System alerts and updates
Documentation: Email-based workflows

Security Considerations

Email Threats

Security Risks:
- Phishing attacks via spoofed headers
- Malware in email attachments
- Script injection in HTML content
- Header injection vulnerabilities

Validation and Sanitization

# Email validation example
import re
from email.utils import parseaddr

def validate_email_header(header_value):
    """Validate email header for security"""
    # Check for header injection
    if '\n' in header_value or '\r' in header_value:
        raise ValueError("Header injection detected")
    
    # Validate email format
    name, addr = parseaddr(header_value)
    email_pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    if not re.match(email_pattern, addr):
        raise ValueError("Invalid email format")
    
    return True

Best Practices

File Handling

Always validate EML file structure before processing
Use proper character encoding (UTF-8 recommended)
Handle large attachments efficiently
Implement proper error handling for malformed files

Content Processing

Sanitize HTML content to prevent XSS attacks
Validate attachment types and sizes
Use virus scanning for attachments
Implement proper spam filtering

Storage and Organization

Use consistent naming conventions for archived emails
Implement compression for large email archives
Maintain proper backup procedures
Consider database storage for searchable archives

Troubleshooting

Common Issues

Parsing Problems:
- Encoding issues with non-ASCII characters
- Malformed MIME boundaries
- Missing or invalid headers
- Corrupted attachment data

Debugging Techniques

# Debug EML parsing
import email
import traceback

def debug_eml_file(filename):
    try:
        with open(filename, 'rb') as f:
            msg = email.message_from_bytes(f.read())
        
        print(f"Message parsed successfully")
        print(f"Headers: {len(msg.keys())}")
        print(f"Multipart: {msg.is_multipart()}")
        
        if msg.is_multipart():
            print(f"Parts: {len(list(msg.walk()))}")
            
    except Exception as e:
        print(f"Error parsing EML: {e}")
        traceback.print_exc()

The EML format provides a standardized way to store and exchange email messages, supporting rich content, attachments, and complex message structures while maintaining human-readable headers for debugging and analysis.

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