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How to Digitally Sign or Encrypt Emails in Outlook Online

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Outlook Online lets you digitally sign or encrypt your emails, either individually or by default for all outbound messages. If this is something you need to do, here are the instructions to get it up and running.

The reasons why you might want to digitally sign an email—applying an electronic signature that proves the message came from the sender that it says it came from—or encrypt an email is a whole topic on its own. We’re going to assume that if you’re interested in digitally signing or encrypting an email you have a good reason for doing so and understand the pros and cons already.

If you don’t know much about digital signing or encryption, then you probably don’t need to worry about either of them. There’s little point signing or encrypting your messages if you don’t need to, and in the case of encryption, you’re just making it so that your recipients won’t be able to read your email.

But, if signing and encrypting email is something you need, here’s how to do it in Outlook Online.

The most important thing to know about signing and encrypting emails in Outlook Online is that you need to be using the Edge browser and an email account that uses Microsoft Exchange. If you’re using Outlook to read your email from Gmail, Yahoo Mail, or any provider that doesn’t use an Exchange server, this won’t work. Similarly, trying this in Chrome, Firefox, or Safari won’t work.

RELATED: What You Need to Know About the New Microsoft Edge Browser

To get started, open the Microsoft Edge browser, and log in to Outlook Online. Open a new email, click the three dots in the taskbar to open a drop-down menu, then select “Show Message Options.”

The "Show message options" option.

In the “Message Options” panel that opens, choose whether you want to sign or encrypt (or both) your message, then click the “OK” button.

The Message options panel, with the Encryption and Digital Signing options highlighted.

If this is the first time you’ve tried signing or encrypting an email on this computer, you’ll see a message that reads “You cannot sign or encrypt this message until you install the S/MIME control. To install S/MIME, click here.” Click the link to install the S/MIME control.

The message stating that the S/MIME control needs to be installed.

You can run the installer directly or download it to your computer and run the installer from there. Assuming you run the installer directly, Edge will ask you for confirmation that you want to install the S/MIME control. Click the “Install” button.

The S/MIME Control installation panel.

Once the S/MIME control has installed, Edge will display a message on the bottom right-hand side of the window.

The message Edge displays when the S/MIME control has been installed.

Close Edge and then reopen the browser. For reasons known only to Microsoft, Edge may not recognize that the S/MIME control is a Microsoft add-in, so click the three dots found in the top-right corner of the interface to open the browser menu and then select the “Extensions” option.

The 3 dots and browser menu in Edge, with the "Extensions" option highlighted.

Find the “Microsoft S/MIME Control” extension and, if it’s set to “Off,” click the toggle to turn it on.

The S/MIME Control on/off toggle switch.

Now, log back into Outlook Online and create a new email. Open the Message Options again (three dots > Show Message Options) and choose to encrypt or sign your email. If you are using a domain that has not been registered with the S/MIME control, you’ll see a message telling you to add the domain. Click the link to open the S/MIME control options.

The "add domain" error message.

In the “S/MIME Control Options” panel, check the box next to “Allow work or school domains,” add your email domain in the text box, then click the “Save” button.

The "S/MIME Control options" panel.

You will now need to restart Edge again and log back in to Outlook Online. You can now send signed and encrypted emails and also unencrypt emails that have been sent to you using the S/MIME control.

If you want to sign or encrypt every email you send by default, click on Settings > View All Outlook Settings.

Outlook's "View all Outlook settings" option.

Click Email > S/MIME.

Outlook's Settings menu, with the "S/MIME" option highlighted.

Switch on either or both of the encryption and signing options, then click the “Save” button.

The S/MIME settings.

All of the emails that you send will now be automatically encrypted and/or signed, depending on which options you chose.

Rob Woodgate Rob Woodgate
Rob Woodgate is a writer and IT consultant with nearly 20 years of experience across the private and public sectors. He's also worked as a trainer, technical support person, delivery manager, system administrator, and in other roles that involve getting people and technology to work together.
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