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Converts Microsoft Outlook .msg files to .eml (email) format using the extract_msg library, preserving email headers, body content (plain text and HTML), and attachments.

Purpose

This function provides an alternative method for converting .msg files to .eml format when standard conversion methods fail. It uses extract_msg's built-in save_email method if available, or manually constructs a MIME-compliant .eml file with proper headers, multipart boundaries, and base64-encoded attachments. This is useful for email migration, archival, or integration with systems that require .eml format.

Source Code

def msg_to_eml_alternative(msg_path, eml_path):
    """Alternative conversion approach using extract_msg's built-in functionality"""
    try:
        if not os.path.exists(msg_path):
            logger.error(f"Input file not found: {msg_path}")
            return False
            
        # Load the .msg file
        logger.info(f"Using alternative conversion method for: {msg_path}")
        msg = extract_msg.Message(msg_path)
        
        # Try direct raw EML content extraction if available
        if hasattr(msg, 'save_email'):
            msg.save_email(eml_path)
            logger.info(f"Successfully converted '{msg_path}' to '{eml_path}' using built-in save_email")
            return True
            
        # Use extract_msg's built-in properties to manually create the EML
        with open(eml_path, 'w', encoding='utf-8') as f:
            # Write basic headers
            f.write(f"From: {msg.sender}\n")
            f.write(f"To: {msg.to}\n")
            if msg.cc:
                f.write(f"Cc: {msg.cc}\n")
            f.write(f"Subject: {msg.subject or ''}\n")
            
            # Add date
            if hasattr(msg, 'date') and msg.date:
                try:
                    f.write(f"Date: {msg.date}\n")
                except:
                    f.write(f"Date: {formatdate(localtime=True)}\n")
            else:
                f.write(f"Date: {formatdate(localtime=True)}\n")
                
            # Add content type header for MIME message
            f.write("MIME-Version: 1.0\n")
            
            # Create a simple multipart message
            boundary = "----=_NextPart_" + os.urandom(16).hex()
            f.write(f'Content-Type: multipart/mixed; boundary="{boundary}"\n\n')
            
            # Add message separator
            f.write(f"--{boundary}\n")
            
            # Add plain text body
            f.write('Content-Type: text/plain; charset="utf-8"\n')
            f.write('Content-Transfer-Encoding: quoted-printable\n\n')
            f.write(msg.body or '')
            f.write(f"\n\n--{boundary}\n")
            
            # Add HTML body if available
            html_content = None
            if hasattr(msg, 'htmlBody') and msg.htmlBody:
                html_content = msg.htmlBody
            elif hasattr(msg, 'html') and msg.html:
                html_content = msg.html
                
            if html_content:
                f.write('Content-Type: text/html; charset="utf-8"\n')
                f.write('Content-Transfer-Encoding: quoted-printable\n\n')
                f.write(html_content)
                f.write(f"\n\n--{boundary}\n")
            
            # Add attachments
            for attachment in msg.attachments:
                try:
                    # Get filename
                    filename = getattr(attachment, 'longFilename', None) or getattr(attachment, 'shortFilename', None) or 'attachment'
                    
                    # Determine content type
                    content_type = None
                    if hasattr(attachment, 'mimetype') and attachment.mimetype:
                        content_type = attachment.mimetype
                    else:
                        content_type, _ = mimetypes.guess_type(filename)
                        
                    if not content_type:
                        content_type = 'application/octet-stream'
                        
                    # Write attachment headers
                    f.write(f'Content-Type: {content_type}; name="{filename}"\n')
                    f.write('Content-Transfer-Encoding: base64\n')
                    f.write(f'Content-Disposition: attachment; filename="{filename}"\n\n')
                    
                    # Write base64 encoded attachment data
                    import base64
                    if attachment.data:
                        encoded_data = base64.b64encode(attachment.data).decode('ascii')
                        # Write in chunks of 76 characters for proper base64 format
                        for i in range(0, len(encoded_data), 76):
                            f.write(encoded_data[i:i+76] + '\n')
                    
                    f.write(f"\n--{boundary}\n")
                    
                except Exception as e:
                    logger.error(f"Error processing attachment {filename}: {str(e)}")
            
            # Close the multipart message
            f.write(f"--{boundary}--\n")
            
        logger.info(f"Successfully converted '{msg_path}' to '{eml_path}' using manual alternative method")
        return True
            
    except Exception as e:
        logger.error(f"Error in alternative conversion of {msg_path} to EML: {str(e)}")
        logger.error(traceback.format_exc())
        return False

Parameters

Parameter Details

msg_path: String path to the input .msg file to be converted. Must be a valid file path pointing to an existing Microsoft Outlook .msg file.

eml_path: String path where the output .eml file will be saved. The directory must exist and be writable. If the file exists, it will be overwritten.

Return Value

Returns a boolean value: True if the conversion was successful, False if any error occurred during the conversion process (e.g., file not found, parsing errors, write errors).

Dependencies

• extract_msg
• os
• mimetypes
• logging
• email
• traceback
• base64

Required Imports

import extract_msg
import os
import mimetypes
import logging
import traceback
from email.utils import formatdate

Conditional/Optional Imports

These imports are only needed under specific conditions:

import base64

Usage Example

import logging
import extract_msg
import os
import mimetypes
import traceback
from email.utils import formatdate

# Setup logger
logger = logging.getLogger(__name__)
logging.basicConfig(level=logging.INFO)

# Convert a .msg file to .eml
msg_file = '/path/to/email.msg'
eml_file = '/path/to/output.eml'

success = msg_to_eml_alternative(msg_file, eml_file)

if success:
    print(f'Successfully converted {msg_file} to {eml_file}')
else:
    print(f'Conversion failed. Check logs for details.')

Best Practices

• Ensure the logger object is properly configured before calling this function
• Verify that the input .msg file exists and is readable before calling
• Check the return value to determine if conversion was successful
• Handle the case where the output directory may not exist by creating it beforehand
• Be aware that this function writes files with UTF-8 encoding, which may cause issues with certain binary content if not properly base64-encoded
• The function attempts to use extract_msg's built-in save_email method first, falling back to manual construction if unavailable
• Large attachments will be loaded into memory during base64 encoding, so ensure sufficient memory is available
• The function uses os.urandom() to generate MIME boundaries, ensuring uniqueness across conversions
• Error handling is comprehensive but errors are logged rather than raised, so check logs for detailed error information

Tags

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