outlook-encrypt-decrypt-messages

title	Encrypt and decrypt messages in Outlook
page_type	sample
urlFragment	outlook-encrypt-decrypt-messages
products	office-outlook office m365
languages	javascript
extensions	contentType technologies createdDate samples Add-ins 01/27/2026 10:00:00 AM
description	Learn how to create an Outlook add-in that encrypts and decrypts messages.

Encrypt and decrypt messages in Outlook (preview)

Applies to: Outlook on Windows (classic)

Summary

Implement custom encryption and decryption functionality in an Outlook add-in to secure email communications. The sample shows how to:

• Encrypt message content and attachments before sending using the OnMessageSend event.
• Automatically identify and decrypt encrypted messages when the recipient opens them using the OnMessageRead event.

To learn about key components of this sample, see Create an encryption Outlook add-in.

Note

• The OnMessageRead event is in preview. Features in preview shouldn't be used in production add-ins.
• This sample uses simplified encryption and decryption protocols for demonstration purposes only. Don't use these protocols in production add-ins.

Applies to

• Classic Outlook on Windows starting in Version 2510 (Build 19312.20000)

Prerequisites

• A Microsoft 365 subscription.

  Note: If you don't have a Microsoft 365 subscription, you might qualify for a free developer subscription that's renewable for 90 days and comes configured with sample data. For details, see the Microsoft 365 Developer Program FAQ.

• To preview the OnMessageRead event in classic Outlook on Windows:

  1. Join the Microsoft 365 Insider program and choose the Beta Channel in the Outlook client. Your client must be on Version 2510 (Build 19312.20000) or later.
  2. Configure your computer's registry to reference the local beta copy of the Office.js API:
     1. In the registry, navigate to HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Options\WebExt\Developer. If the key doesn't exist, create it.
     2. Create an entry named EnableBetaAPIsInJavaScript and set its value to 1.

• Install a recent version of npm and Node.js on your computer if you want to run the web server on localhost. To check if you've already installed these tools, from a command prompt, run the following commands.

  node -v
  npm -v

Run the sample

To host the web server on localhost, follow these steps.

1. Clone or download this repository.

2. From a command prompt, go to the root of the project folder /samples/outlook-encrypt-decrypt-messages.

3. Run the following commands.

   npm install
   npm run build
   npm start

   This starts the web server on localhost and sideloads the manifest.xml file to Outlook.

4. Follow the steps in Try it out to test the sample.

5. To stop the web server and uninstall the add-in from Outlook, run the following command.

   npm stop

Try it out

Once the add-in is loaded in Outlook, use the following steps to try out its functionality.

Encrypt a message

1. In classic Outlook on Windows, create a new message.

2. Add recipients, a subject, and content to the message body.

   Note: The recipient must also have the add-in installed to decrypt the message. To test this sample, send the message to yourself.

3. Optionally, add attachments to the message.

   Note: The decryption API doesn't currently support attachments of type Office.MailboxEnums.AttachmentType.Item.

4. Select Send.

   When the OnMessageSend event occurs, the encrypted message body and attachments are added as .txt files to the message. The body is replaced with a placeholder message and the original attachments are removed from the message.

   

Decrypt a message

Select the encrypted message from the inbox.

The OnMessageRead event occurs and decrypts the message content and its attachments. The following notification is displayed: "Office Add-ins Sample Encryption Add-in is decrypting your message".

Once decryption is complete, the message body and attachments are shown and the following notification is displayed: "Office Add-ins Sample Encryption Add-in has decrypted your message".

Questions and feedback

• Did you experience any problems with the sample? Create an issue and we'll help you out.
• We'd love to get your feedback about this sample. Go to our Office samples survey to give feedback and suggest improvements.
• For general questions about developing Office Add-ins, go to Microsoft Q&A using the office-js-dev tag.

Copyright

Copyright (c) 2026 Microsoft Corporation. All rights reserved.

This project has adopted the Microsoft Open Source Code of Conduct. For more information, see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Solution

Solution	Author(s)
Encrypt and decrypt messages in Outlook	Microsoft

Version history

Version	Date	Comments
1.0	January 27, 2026	Initial release