Email File Formats

EML Files Explained: Deep Dive Into Single-Message Email Storage and Analysis

By MDToolsOne •
EML single email message file structure Understanding how individual email messages are stored in EML files

EML files represent a single, complete email message stored in a plain-text format. Unlike mailbox formats such as MBOX, which store many messages together, EML focuses on one email per file.

EML is widely used for message exports, forensic investigations, email clients, and debugging delivery or rendering issues. Understanding EML is essential for developers, security analysts, and anyone working deeply with email systems.

What Is an EML File?

An EML file contains the raw RFC-compliant representation of a single email, including headers, body, and attachments.

  • One email message per file
  • Plain text, human-readable
  • Uses standard RFC 5322 and MIME formats
  • Supported by most email clients and tools

EML File Structure

An EML file is divided into two main sections:

1. Headers

From: sender@example.com
To: user@example.com
Subject: Test message
Date: Sat, 21 Dec 2025 15:22:10 +0000
Message-ID: <abc123@example.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8

Headers describe metadata such as sender, recipient, routing information, authentication results, and content type.

2. Body

After a blank line, the message body begins. This may contain:

  • Plain text
  • HTML content
  • Multipart MIME sections
  • Base64-encoded attachments

MIME and Attachments in EML

EML files rely heavily on MIME (Multipurpose Internet Mail Extensions) to support rich content and attachments. 

Content-Type: multipart/mixed; boundary="abc123"

Each MIME part can represent:

  • Text or HTML body
  • Inline images
  • File attachments

Common Use Cases for EML Files

  • Email client exports (Outlook, Thunderbird, Apple Mail)
  • Customer support and ticketing systems
  • Spam and phishing investigations
  • Legal and compliance archiving
  • Debugging rendering or encoding issues

EML vs MBOX

Feature EML MBOX
Storage model One message per file Multiple messages in one file
Ease of parsing High Moderate
Concurrency safety Excellent Poor
Best for Individual analysis Bulk storage

Parsing EML Files Programmatically

Because EML files follow standard email formats, they can be parsed safely using mature libraries.

Common Tools

  • Python: email module
  • Node.js: mailparser
  • CLI: munpack, ripmime
Always rely on a standards-compliant parser — never parse EML files manually.

Security Considerations

EML files often contain sensitive data and should be handled carefully:

  • Attachments may contain malware
  • Headers may expose internal infrastructure
  • Embedded links may be malicious

When analyzing EML files, use sandboxed environments and never open untrusted attachments directly.

When to Use EML

EML is ideal when:

  • You need to store or share a single message
  • Investigating spam or phishing incidents
  • Exporting messages from email clients
  • Debugging MIME, headers, or encoding issues

Final Thoughts

EML files provide a transparent, standards-based view into how individual emails are constructed and delivered.

For developers and security professionals, mastering EML is essential for debugging, analysis, and understanding email at its most fundamental level.

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