Explain MSG & EML File Formats? How to Convert b/t Them via .NET, Java & Python?

Last Updated: 06 Aug, 2025

When working with Email file formats, you may encounter two common formats: MSG and EML. Both store email messages but differ in structure, compatibility, and usage. In this blog post, we’ll explore what they are, their features and differences, and how to convert between MSG and EML using various types of APIs and code examples.

What is an MSG File?

The MSG file format is a proprietary format developed by Microsoft. It is a single email message, appointment, contact, or task saved from Microsoft Outlook. It’s a proprietary format, meaning it’s designed to work best within the Outlook ecosystem. MSG files hold all the details of an email, including the sender, recipient, date, subject, body, and attachments, as well as Outlook-specific features like categories and flags.

Key Features:

• Stores email content, attachments, and metadata (sender, recipient, subject).
• Binary format based on Compound File Binary Format (CFBF).
• Stores metadata, email body (RTF or HTML), and attachments.
• Used primarily in Windows environments.

What is an EML File?

The EML file format is an open standard defined in RFC 5322, and is a more universal email format used by various email clients like Mozilla Thunderbird, Windows Mail, Outlook Express, and Apple Mail. Because it’s a plain text format, it’s more versatile and can be opened on different operating systems and devices.

Key Features:

• Text-based format that stores headers and MIME-encoded content.
• Fully supports HTML formatting, inline images, and attachments.
• Platform-independent and human-readable.
• Easier to process programmatically.

Key Differences between MSG and EML

Now let’s explore how to convert MSG to EML and EML to MSG using popular open source APIs and libraries. Below are code examples using different APIs.

How to Convert MSG to EML using .NET APIs?

We can use open source libraries MsgKit and MimeKit for converting MSG file to EML file format inside .NET applications. While MsgKit alone doesn’t offer native .eml export, it makes it easy to read and extract all components of a .msg file, which can then be written out as .eml using other MIME-compliant libraries like MimeKit or MailKit as shown in the following code example

How to Convert MSG File to EML using MsgKit?

using MsgReader.Outlook;
using MimeKit;
using System.IO;

var msg = new Storage.Message("sample.msg");
var mimeMessage = new MimeMessage();
mimeMessage.Subject = msg.Subject;
mimeMessage.From.Add(new MailboxAddress(msg.SenderName, msg.SenderEmail));
mimeMessage.To.Add(MailboxAddress.Parse(msg.GetEmailRecipients(Storage.RecipientType.To)[0].Email));
mimeMessage.Body = new TextPart("plain") { Text = msg.BodyText };

using var stream = File.Create("output.eml");
mimeMessage.WriteTo(stream);

How to Convert EML to MSG File using MimeKit?

using MsgKit;
using System.IO;

var email = new Email(
    new Sender("sender@example.com", "Sender Name"),
    "Subject of Email",
    "This is the plain body text.");

email.Recipients.AddTo("recipient@example.com", "Recipient Name");
email.Save(new FileInfo("output.msg"));

Convert MSG to EML via Open Source Java APIs?

Apache POI-HSMF and JavaMail can be used to load and convert Outlook MSG to EML file format. Apache POI-HSMF can read MSG files, and JavaMail can generate EML files. Conversion from EML to MSG in Java would typically require either paid SDKs (like Aspose) or manual MIME parsing and MSG binary writing—not currently feasible using purely open-source Java libraries.

The following code example demonstrates how Apache POI-HSMF can read MSG files, and JavaMail can generate EML files.

import org.apache.poi.hsmf.MAPIMessage;
import javax.mail.*;
import javax.mail.internet.*;

public class MsgToEmlConverter {
    public static void convert(String msgPath, String emlPath) throws Exception {
        MAPIMessage msg = new MAPIMessage(msgPath);
        MimeMessage eml = new MimeMessage((Session) null);
        
        eml.setFrom(new InternetAddress(msg.getDisplayFrom()));
        eml.setSubject(msg.getSubject());
        eml.setText(msg.getTextBody());
        
        // Save EML file
        eml.writeTo(new java.io.FileOutputStream(emlPath));
    }

    public static void main(String[] args) throws Exception {
        convert("input.msg", "output.eml");
    }
}

Best Commercial APIs for High-Performance Conversion

1. Aspose.Email (Cross-Platform .NET/Java/Python)

Aspose.Email offers a powerful commercial solution to work with email files. With just a couple of lines of code software developers can load, manipulate and convert MSG to EML and EML to MSG using various programming languages like.NET, Java, Python and many more. The following example shows how to convert b/t MSG and EML using Aspose.Email for .NET API.

How to Convert b/t EML to MSG using Aspose.Email for .NET API?

// Convert MSG to EML
using Aspose.Email.Mime;
using Aspose.Email.Outlook;

var msg = MapiMessage.FromFile("input.msg");
msg.Save("output.eml", SaveOptions.DefaultEml);

// Convert EML to MSG
var eml = MailMessage.Load("input.eml");
eml.Save("output.msg", SaveOptions.DefaultMsgUnicode);

How to Convert b/t EML to MSG using Aspose.Email for Java API?

import com.aspose.email.*; // Load the MSG file MailMessage message = MailMessage.load("input.msg"); // Save as EML message.save("output.eml", SaveOptions.getDefaultEml());

How to Convert b/t EML to MSG using Aspose.Email for Python API?

import aspose.email as ae

# Load the EML file
eml = ae.MailMessage.load("input.eml")

# Save as MSG
eml.save("output.msg", ae.SaveOptions.default_msg_unicode)

2. Microsoft Graph API

Another good commercial option to consider it Microsoft Graph API (Cloud-based)

Convert MSG to EML via Microsoft Graph API (Cloud-based)

import requests

headers = {"Authorization": "Bearer YOUR_ACCESS_TOKEN"}
response = requests.post(
    "https://graph.microsoft.com/v1.0/me/messages/{id}/content?format=eml",
    headers=headers
)
with open("output.eml", "wb") as f:
    f.write(response.content)

Bonus: Online Converters (No Code)

If you don’t want to code, here is a very reliable and useful online tool that easily load and convert MSG file to EML format.

• Aspose Free Converter

Final Thoughts

Both MSG and EML formats have their unique strengths. MSG is ideal for Microsoft ecosystems with full metadata and embedded attachments, while EML offers greater flexibility due to its plain-text, MIME-based structure. Whether you’re a .NET, Java, Python, or JavaScript developer, there are open-source and commercial tools to convert between MSG and EML formats. Choose based on your language, project scale, and licensing need

FAQ

Q: Can I open MSG files without Outlook?

A: Yes, open-source libraries like MsgReader (C#) or extract-msg (Python) allow reading MSG files without Outlook.

Q: Which format is better for email archival?

A: EML is better due to its open standard and human-readable format.

Q: Are there free APIs to convert MSG to EML?

A: Yes, libraries like MsgReader (.NET) and extract-msg (Python) are completely free and open source.

See Also

• Email File Formats at FileFormat.com?
• How Do I Convert a PDF to FDF?
• What is the Difference Between PDF and FDF?
• PDF vs Word: Which One Should You Use and When?
• .h vs .hpp: What’s the Difference and Which Should You Use?