Aspose.Email  for .NET

eSign OST Formats in C#

Native and high performance OST message electronic signature using server-side Aspose.Email for .NET APIs, without the use of any software like Microsoft or Open Office, Adobe PDF.

How to Digitally Sign OST File Using C#

In order to electronically sign OST file, we’ll use

Aspose.Email for .NET

API which is a feature-rich, powerful and easy to use digital signature API for C# platform. Open


package manager, search for Aspose.Email and install. You may also use the following command from the Package Manager Console.


PM> Install-Package Aspose.Email

Steps for e-Signing OST Files in C#

A basic message digital signature with

Aspose.Email for .NET

APIs can be done with just few lines of code. One can sign email processing documents by adding text, image or draw professional digital signature to the email files as well as save result in MSG and EML formats.

  • Load all the OST files with full path.
  • Set the style of text like font, font size, color etc.
  • Set the Signature text.
  • Save OST file, You will get the document with signature inserted.

System Requirements

Our APIs are supported on all major platforms and Operating Systems. Before executing the code below, please make sure that you have the following prerequisites on your system.

  • Microsoft Windows or a compatible OS with .NET Framework, .NET Core, and Xamarin Platforms
  • Development environment like Microsoft Visual Studio
  • Aspose.Email for .NET DLL referenced in your project - Install from NuGet using the Download button above

e-Sign OST Files - C#

var mail = MapiHelper.GetMapiMessageFromFile(inputFilePath);

var htmlDocument = new Aspose.Html.HTMLDocument(mail.BodyHtml, "");

var element = htmlDocument.CreateElement("Signature");
element.InnerHTML = "Signature Text (Style it with css)";

var folderPath = "directorypath";
var filePath = folderPath + "Merged.html";
var content = System.IO.File.ReadAllText(filePath);
mail.SetBodyContent(content, BodyContentType.Html);

mail.Save(Path.Combine(outputFolderPath, Path.GetFileNameWithoutExtension(inputFilePath) + " Signed.ost"), new EmlSaveOptions(MailMessageSaveType.OstFormat));  
  • About Aspose.Email for .NET API

    Aspose.Email is a Microsoft Outlook and Thunderbird formats parsing solution. One can easily create, manipulate, convert email and storage formats such as MSG, EMLX, EML and MHT. Handling of email attachments, customization of message headers and implementation of different network protocols like POP3, IMAP & SMTP to send & receive emails is much easier. Its a standalone API and does not require Microsoft Outlook or any other software installation.

    Online OST eSignature Live Demos

    OST What is OST File Format

    OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from email server. OST files can user mailbox items such as emails, contacts, calendar information, notes, tasks and other similar data. Users can create emails and other data items in OST file even in the absence of connection to the server, but these will not be synchronized with the server. Once the connection is established, the local file is synchronized with the server again so that both the server and the local copy are at the same level of information.

    Read More

    Other Supported Files for Electronic Signatures

    Using C#, One can also digitally sign many other formats including.

    EML (Outlook Email Messages)
    MBOX (Electronic Mail Messages)
    MSG (Outlook & Exchange Format)
    PST (Outlook Personal Storage Files)