Introduction

Aspose.Email FOSS for .NET is a free, open-source C# library that lets you work with Outlook MSG files, Compound File Binary (CFB) containers, and EML messages directly in your .NET application — no Microsoft Outlook, no COM interop, and no native libraries required. Distribute it as a single NuGet package and it runs identically on Windows, Linux, macOS, Docker containers, and serverless functions.

The library solves a recurring problem in enterprise and integration work: Outlook .msg files are produced by millions of business users, but parsing them without Outlook has historically required COM automation or expensive commercial libraries. Aspose.Email FOSS for .NET provides full read/write support for MSG, the underlying CFB binary container, and the EML/MIME format — all under the MIT license with no usage restrictions.

Whether you are building an email archival pipeline, a migration tool, an attachment extractor, or an automated MSG generator, this library gives you direct, low-level access to every byte of the email format alongside a high-level MapiMessage API that lets you get started in minutes.

Key Features

Read Outlook MSG Files

Open any .msg file from a path or stream with MsgReader.FromFile() or MsgReader.FromStream(). Access the complete MAPI property stream, recipient records, and attachment sub-storages at the raw byte level via MsgStorage and MsgStream.

using System.IO;
using Aspose.Email.Foss.Msg;
using var stream = File.OpenRead("sample.msg");

var message = MapiMessage.FromStream(stream);
Console.WriteLine(message.Subject);

Create MSG Files from Scratch

Build a complete email programmatically using MapiMessage.Create(). Set the subject, plain-text body, HTML body, sender name, sender email address, delivery time, and internet message ID, then add recipients with AddRecipient() and attachments with AddAttachment(). Serialize with message.Save().

using System.IO;
using Aspose.Email.Foss.Msg;

var message = MapiMessage.Create("Hello", "Body");
message.SenderName = "Alice";
message.SenderEmailAddress = "alice@example.com";
message.AddRecipient("bob@example.com", "Bob");
using var attachmentStream = new MemoryStream("abc"u8.ToArray());
message.AddAttachment("note.txt", attachmentStream, "text/plain");
using var output = File.Create("hello.msg");
message.Save(output);

Convert EML to MSG (and Back)

Load a standard RFC 5322 .eml file into a MapiMessage with MapiMessage.LoadFromEml(), then save it as an Outlook .msg file — or reverse the process with MapiMessage.SaveToEml(). The built-in MIME parser preserves subject, body, HTML body, sender, recipients, and attachments through full EML ↔ MSG round-trips.

using System.IO;
using Aspose.Email.Foss.Msg;
using var input = File.OpenRead("message.eml");

var message = MapiMessage.LoadFromEml(input);
using var msgOutput = File.Create("message.msg");
message.Save(msgOutput);
using var emlOutput = File.Create("roundtrip.eml");
message.SaveToEml(emlOutput);

Low-Level CFB Container Access

Outlook MSG files are built on the Compound File Binary (CFB) format — a hierarchical binary container similar to a filesystem. CfbReader exposes the full directory tree: iterate storages and streams with IterStorages(), IterStreams(), and IterChildren(), navigate to specific entries by path with ResolvePath(), and read raw stream bytes with GetStreamData().

CfbWriter lets you construct and serialize a CFB document from scratch. Create a CfbDocument, attach CfbStorage and CfbStream nodes, and call CfbWriter.ToBytes() or CfbWriter.WriteFile() to serialize.

MAPI Property and Attachment Access

The MapiPropertyCollection class provides typed MAPI property access for any MSG or storage object: use Get(), Set(), Add(), and Remove() with standard CommonMessagePropertyId or PropertyTypeCode enum values. Access attachment metadata and binary data via MapiAttachment properties: Filename, MimeType, ContentId, and Data.

Quick Start

Install the package:

dotnet add package Aspose.Email.Foss

Read a subject from an MSG file:

using System.IO;
using Aspose.Email.Foss.Msg;

using var stream = File.OpenRead("sample.msg");
var message = MapiMessage.FromStream(stream);
Console.WriteLine(message.Subject);
Console.WriteLine(message.SenderEmailAddress);
foreach (var recipient in message.Recipients)
    Console.WriteLine($"To: {recipient.EmailAddress}");
foreach (var attachment in message.Attachments)
    Console.WriteLine($"Attachment: {attachment.Filename} ({attachment.MimeType})");

Supported Formats

FormatExtensionReadWrite
Compound File Binary.cfb✓✓
Outlook Message.msg✓✓
MIME / EML.eml✓✓

Open Source & Licensing

Aspose.Email FOSS for .NET is released under the MIT license. You may use it freely in personal, commercial, and open-source projects with no usage restrictions. The source code is available at github.com/aspose-email-foss/Aspose.Email-FOSS-for-.Net.

Getting Started