Aspose.Email FOSS
.NET
How to Convert EML to MSG in C#

How to Convert EML to MSG in C#

This guide shows how to convert between EML (RFC 5322 / MIME) and Outlook MSG formats using Aspose.Email FOSS for .NET.

Step 1 — Install the Package 

dotnet add package Aspose.Email.Foss

Step 2 — Convert EML to MSG 

using Aspose.Email.Foss.Msg;

var message = MapiMessage.LoadFromEml("incoming.eml");
message.Save("converted.msg");

You can also load from a stream or byte array:

// From stream
using var stream = File.OpenRead("incoming.eml");
var message = MapiMessage.LoadFromEml(stream);
message.Save("converted.msg");

// From byte array
var emlData = File.ReadAllBytes("incoming.eml");
var message2 = MapiMessage.LoadFromEml(emlData);
message2.Save("converted.msg");

Step 3 — Convert MSG to EML 

using var message = MapiMessage.FromFile("original.msg");

// To file
message.SaveToEml("exported.eml");

// To byte array
byte[] emlBytes = message.SaveToEml();

Step 4 — Verify Round-Trip Fidelity

Subject, body, sender, recipients, and attachments survive the round-trip:

// EML → MSG → EML
var original = MapiMessage.LoadFromEml("test.eml");
original.Save("test.msg");

using var reloaded = MapiMessage.FromFile("test.msg");
reloaded.SaveToEml("roundtrip.eml");

Console.WriteLine($"Subject match: {original.Subject == reloaded.Subject}");
Console.WriteLine($"Attachments match: {original.Attachments.Count == reloaded.Attachments.Count}");

Batch Conversion Example

Convert all .eml files in a directory to .msg:

using Aspose.Email.Foss.Msg;

foreach (var emlFile in Directory.GetFiles("inbox", "*.eml"))
{
    var message = MapiMessage.LoadFromEml(emlFile);
    var msgFile = Path.ChangeExtension(emlFile, ".msg");
    message.Save(msgFile);
    Console.WriteLine($"Converted: {emlFile} → {msgFile}");
}
 English