This update fixes the following issues that you may experience when you use the S/MIMIE encryption in the Outlook on the web client (previously known as Outlook Web App) in Microsoft Exchange Server 2016:
When you access a mailbox that's hosted on a computer by using the Outlook on the web client that has the S/MIME control installed, you may receive the error message "Your S/MIME control is out of date. You must reinstall it." In addition, this issue persists even if you reinstall the S/MIME control.
You can't send an S/MIME signed or encrypted email message in the Outlook on the web client. In addition, you may receive the error message "An error occurred while encoding this S/MIME message. An unexpected S/MIME error occurred." especially when you rehydrate a draft, compose the email message for more than a minute, or add attachments or images to the message.
How to get this update
After you install the S/MIME control update, when you open the Outlook on the web client in Internet Explorer for the first time, you are prompted with the following message:
Do you want to allow the domain <owa website> to use the S/MIME control to encrypt and decrypt messages in your inbox? Only allow domains that you trust. If you select Yes, you won't be prompted again for this domain. If you select No, the domain won't have access to S/MIME.
When it's shown, click Yes to trust the domain.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
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