.EML File Extension Reference
EML is an email archive file format developed by Microsoft for Outlook Express and standardized under RFC 822. It contains raw Internet email headers, plain text body, HTML body, and MIME encoded file attachments.
Quick Specification & Metadata
.EML
Email & Comm
message/rfc822
IETF (RFC 822 Standard)
1982
Proprietary / Patent Licensed
Binary Header Data
15 KB – 10 MB
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What is a .EML File? (Technical Definition & Architecture)
EML is an email archive file format developed by Microsoft for Outlook Express and standardized under RFC 822. It contains raw Internet email headers, plain text body, HTML body, and MIME encoded file attachments.
Formatted byte alignment adhering to international format specifications.
Adheres to formal schema and protocol documentation.
Parsed and executed identically across Windows, macOS, and Linux kernels.
EML Binary Structure & Header Specifications
The .EML format organizes data into validated sequential or hierarchical byte structures. Inspecting the header offset reveals the magic byte sequence 46 72 6F 6D 3A (ASCII text: From:).
- File Signature (Magic Bytes): 46 72 6F 6D 3A (ASCII text: From:)
- MIME Content-Type: message/rfc822
- Payload Structure: Optimized data stream with metadata headers and checksum records.
- Safety Classification: Medium Risk – EML files can carry malicious email attachments or phishing links. Exercise caution when opening untrusted EML files.
How to Open .EML Files on Windows, Mac & Android
- 1Primary Application: Double-click or open directly with Mozilla Thunderbird.
- 2Alternative Software: Open using Microsoft Outlook 365.
- 3File Association: Right-click the file, select Open with > Choose another app, and check Always use this app.
- 1Native macOS Application: Open with Mozilla Thunderbird.
- 2Quick Look Preview: Highlight the file in Finder and tap the Spacebar.
- 3Set Default App: Press Cmd + I, expand Open with, select your application, and click Change All.
- 1Compatible Mobile Apps: Download a dedicated viewer for .eml from the Google Play Store.
- 2Cloud Storage: Open through Google Drive, Dropbox, or OneDrive mobile apps.
- 3Online Browser Conversion: Use AnyFileX mobile browser tools to convert the file into universal PDF or image formats.
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.EML MIME Types & HTTP Server Headers
When serving .eml files over HTTP or configuring REST APIs, web servers must send the official IANA registered Content-Type headers:
| MIME Type Identifier | Standard Usage | File Extensions | Copy |
|---|---|---|---|
| message/rfc822 | Standard official MIME type for .eml files | .eml |
# Nginx mime.types
types {
message/rfc822 eml;
}
# Apache .htaccess
AddType message/rfc822 .eml.EML Magic Bytes & Hex File Signatures
In digital forensics and file signature analysis, authentic .eml files begin with a standardized magic byte sequence at byte offset 0x00 to verify file integrity and prevent extension spoofing:
Offset 0x0046 72 6F 6D 3AEMLEmail & Comm File Format.EML vs .PDF (Detailed Format Comparison)
| Feature Metric | .EML | Advantage | |
|---|---|---|---|
| Format Full Name | RFC 822 Email Message Archive | Portable Document Format | Tie / Equal |
| Category | Email & Comm | Documents | Tie / Equal |
| MIME Content-Type | message/rfc822 | application/pdf | Tie / Equal |
| Binary Header (Magic Bytes) | 46 72 6F 6D 3A (ASCII text: From:) | 25 50 44 46 2D (%PDF-) | Tie / Equal |
| Typical Storage Footprint | 15 KB – 10 MB | 200 KB – 25 MB | Tie / Equal |
| Universal Native Support | High / OS Standard | High / OS Standard | Tie / Equal |
.EML Compatibility Matrix
Supported natively or through Mozilla Thunderbird
Supported natively or via Microsoft Outlook 365
Supported via open-source tools and command line utilities
Accessible through compatible mobile viewers or cloud previewers
Supported through dedicated mobile apps or cloud viewers
Accessible via web browser applications and Chrome extensions
Recommended Software for .EML
Mozilla Thunderbird
Mozilla Foundation
Microsoft Outlook 365
Microsoft
Apple Mail
Apple Inc.
Common Errors & Troubleshooting .EML Files
Security & Malware Risk Assessment
Can a .EML file carry viruses or malware?
EML files can carry malicious email attachments or phishing links. Exercise caution when opening untrusted EML files.
May support scripting or automation macros
Safe to inspect with AnyFileX's browser sandbox
- •Verify file extension in OS File Explorer options ('Show known file extensions').
- •Verify magic bytes (46 72 6F 6D 3A (ASCII text: From:)) to ensure the file isn't a renamed executable.
- •Do not open .EML attachments received unexpectedly via email or instant messenger.
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