MailCrush

A high-efficiency mail lossless compression tool that deconstructs emails for maximum compression.

MailCrush is an EDLA project.

The purpose of edla.org is to promote the state of the art in various domains.

Overview

MailCrush is a command-line tool designed to analyze, compress, and decompress email files with maximum efficiency. It deconstructs emails into their MIME parts, applies content-aware compression algorithms, and can perfectly reconstruct the original email byte-for-byte.

Key Features

• Lossless Compression: Perfect reconstruction of original emails verified through hash comparison
• Content-Aware Compression: Automatically selects the best compression algorithm (LZ4, Zstd, Gzip) based on content type
• Base64 Decoding: Decodes Base64-encoded attachments before compression for better ratios
• MIME Structure Analysis: Deep inspection of email structure including nested multipart messages
• Batch Processing: Process entire directories of emails recursively
• Detailed Statistics: Compression reports with per-part breakdowns

Installation

From Source

Requires Rust 2024 edition :

git clone https://github.com/newca12/mailcrush.git
cd mailcrush
cargo build --release

The binary will be available at target/release/mailcrush.

Usage

Basic Commands

# Show basic information about an email
mailcrush info email.eml

# Analyze email structure in detail
mailcrush analyze email.eml

# List all parts/attachments
mailcrush list email.eml

# Compress an email
mailcrush compress email.eml -o email.mcr

# Decompress a compressed email
mailcrush read email.mcr -o restored.eml

# Show compression statistics
mailcrush stats email.eml

# Validate email structure
mailcrush validate email.eml

# Extract attachments
mailcrush extract email.eml -o ./attachments/

Command Reference

Global Options

• -v, --verbose: Enable verbose output
• -d, --debug: Enable debug output

Compress Options

mailcrush compress <PATH> [OPTIONS]

Options:
  -r, --recursive       Process directories recursively
  -o, --output <PATH>   Output file or directory path
  -l, --level <1-9>     Compression level (default: 6)
  --dry-run             Show what would be compressed without compressing
  -t, --timer           Show timing information

Read Options

mailcrush read <PATH> [OPTIONS]

Options:
  -r, --recursive       Process directories recursively
  -o, --output <PATH>   Output file or directory for decompressed email(s)
  --raw                 Output raw email content
  --headers-only        Show headers only
  -t, --timer           Show timing information

List Options

mailcrush list <PATH> [OPTIONS]

Options:
  -r, --recursive       Process directories recursively
  -a, --attachments     Show only attachments
  -b, --base64          Show only Base64 encoded parts

Extract Options

mailcrush extract <PATH> [OPTIONS]

Options:
  -r, --recursive           Process directories recursively
  -o, --output-dir <PATH>   Output directory for extracted files (default: .)
  -p, --part <INDEX>        Extract specific part by index (1-based)
  -a, --all                 Extract all parts, not just attachments

File Format

MailCrush uses the .mcr extension for compressed email files. The format stores:

• Compressed email parts with their metadata
• Original encoding information for perfect reconstruction
• Content hashes for integrity verification

How It Works

1. Parse: The email is parsed using the mail-parser library to extract MIME structure
2. Analyze: Each part is analyzed for content type, encoding, and compressibility
3. Decode: Base64 and quoted-printable encoded parts are decoded to their raw form
4. Compress: Each part is compressed using the most suitable algorithm:
   • LZ4: Fast compression for text content
   • Zstd: Balanced compression for general content
   • Gzip: Good general-purpose compression
   • None: For already-compressed content (images, archives, etc.)
5. Verify: The original email is reconstructed and hash-verified to ensure lossless compression

Example Compression Report

Here's a real compression report for a sample email with attachments:

================================================================================
📦 COMPRESSION REPORT
================================================================================

📋 EMAIL INFO:
  Subject: Remis à 0198410730: MARDI.xls
  Original size: 246027 bytes (240.26 KB)
  Compressed size: 55988 bytes (54.68 KB)
  Compression ratio: 77.2%

✅ VERIFICATION:
  Full email reconstruction: ✓ VERIFIED
  Parts verified: 9/9

🔍 PART DETAILS:
--------------------------------------------------------------------------------
   # | Content-Type                   | Algorithm  |   Original | Compressed |  Savings | Status
--------------------------------------------------------------------------------
   1 | multipart/mixed                | None       |        0 B |        0 B |     0.0% | ✓
   2 | multipart/related              | None       |        0 B |        0 B |     0.0% | ✓
   3 | multipart/alternative          | None       |        0 B |        0 B |     0.0% | ✓
   4 | text/plain                     | Zstd       |      250 B |      208 B |    16.8% | ✓
   5 | text/html                      | Zstd       |     5265 B |     1236 B |    76.5% | ✓
   6 | image/gif (fax_ok.gif)         | None       |    11928 B |     8822 B |    26.0% | ✓
   7 | image/gif (page1.gif)          | None       |    29687 B |    21959 B |    26.0% | ✓
   8 | application/pdf (91624c3d.pdf) | Zstd       |   108527 B |    10531 B |    90.3% | ✓
   9 | application/pdf (Rapport.pdf)  | Zstd       |    87815 B |    13232 B |    84.9% | ✓
--------------------------------------------------------------------------------

📊 SUMMARY:
  Total savings: 190039 bytes (77.2%)
  Verification: ✓ All parts verified successfully

✅ Compressed email saved to: "123270.mcr"
   Original size:         246027 bytes
   Archive size:          70450 bytes
   Space savings:         175577 bytes (71.4%)

   Structure original:    2555 bytes
   Structure compressed:  880 bytes (65.6% saved)
   Parts original:        243472 bytes
   Parts compressed:      55988 bytes
   Parts compression:     77.0%

This example shows:

• 77.2% compression ratio on the email parts
• 71.4% overall space savings including the archive overhead
• PDF attachments compressed with Zstd achieving up to 90.3% savings
• Lossless verification confirming all 9 parts reconstructed correctly

Dependencies

• mail-parser - Email parsing
• zstd - Zstandard compression
• lz4_flex - LZ4 compression
• flate2 - Gzip compression
• clap - Command-line argument parsing
• sha2 - SHA-256 hashing for verification

License

© 2026 Olivier ROLAND. Distributed under the GPLv3 License.