In order to be able to validate S/MIME digitally signed and/or encrypted emails in Outlook on the Web (OWA) / Exchange Online (Office 365), a Microsoft browser extension is required to be installed, called SMIME Control.
Microsoft has phased out support for Internet Explorer as of Q1 2021.
This S/MIME extension is only available for Edge (user can install it), and for Chrome (only the company admin can make it available through a Group Policy/GPO). For Chrome support, your user’s device MUST be domain joined!!
Edge browser (domain joined is not required):
Chrome browser (domain joined is required):
To have the browser install the Microsoft S/MIME extension, you’ll have to set the ExtensionInstallForceList value in the Registry. Below you’ll find the location for both Microsoft Edge and Google Chrome.
Key Microsoft Edge: HKEY_CURRENT_USER\Software\Policies\Microsoft\Edge\ExtensionInstallForcelist
Key Google Chrome: HKEY_CURRENT_USER\Software\Policies\Google\Chrome\ExtensionInstallForcelist
Value Name: 1
Value type: REG_SZ
Value Office 365: maafgiompdekodanheihhgilkjchcakm;https://outlook.office.com/owa/SmimeCrxUpdate.ashx
Value Exchange On-Premises: maafgiompdekodanheihhgilkjchcakm;https://<your owa url>/owa/SmimeCrxUpdate.ashx
Setting the ExtensionInstallForcelist value in the Registry Editor to install the S/MIME extension.
To set the registry value above, you can also use the Group Policy templates.
For instructions on how to install and use Group Policy templates you can use the guide; Setting Outlook Group Policies.
You can find the policy setting in the following location;
Set the policy setting to Enabled the and click on the “Show…” button to add the following value;
Setting the ExtensionInstallForcelist value in the Group Policy Editor to install the S/MIME extension.
When you are using a mailbox hosted in Office 365 or Outlook.com, you can skip this step.
When you are using a mailbox hosted in an On-Premises Exchange environment, then you must add the Outlook on the Web domain to the “Allowed domains” list.
Infobar message when using the S/MIME Extension in an On-Premises Exchange environment.
To do this, click on the “click here” link in the infobar message or click on the Outlook icon right from the Address Bar and choose: Extension Options.
Another way to get to the Extension Options page of the S/MIME extension is by going to the following page;
Add your On-Premises mail domain to the S/MIME Control options.
After installing the extension, you’ll notice that you still won’t be able to open S/MIME encrypted messages. The infobar however has changed and instructs you to install the S/MIME control and contains a link to download this control.
For Office 365, this link currently point to here but that can change in the future.
For Exchange On-Premises, it points to;
https://<your owa url>/owa/smime/SmimeOutlookWebChrome.msi
This link can also be found in the S/MIME Settings section of the Options page in Outlook on the web. Instructions for getting to this additional options page are in Step 4.
Because this S/MIME control installer is actually intended for deployment, the installation of this package is silent and doesn’t give any feedback whether the installation was successful or not. You can however still install it manually and it doesn’t require administrator permissions to install it for most domain configurations.
Give the installer a good minute to complete. When it installed successfully, it will also show in your Apps list in Windows Settings.
Once installed, restart the browser and you should be able to read and send S/MIME encrypted emails as well as adding digital signatures to emails. The infobar will show the following information when the message can be decrypted and the digital signature can be verified;
Source: https://robert365.com/article/smime-support-for-outlook-on-the-web
