S/MIME compatibility in Office 365 Outlook Web App (OWA)
Messages that are digitally signed and/or encrypted are not displayed in the Office365 Outlook Web App (OWA) when e-mails are viewed with “Conversations” enabled, prompting the message “S/MIME is not supported in current browser or platform.” (S/MIME is the standard used for digital signatures, which provide information on a sender’s identity, and encryption, which ensures only the intended recipients can read a message.)
The Office 365 Team has a support ticket with Microsoft. ITS will provide more information when Microsoft offers a solution.
The workaround for this issue at this time is to turn Conversations off. This will allow digitally signed and encrypted messages to display properly in the Office365 Outlook Web App (OWA).
From Outlook (mail) section of the Outlook Web App (OWA), click on the drop-down list located on the top-right of the list of messages (for a given folder you're viewing, e.g., your Inbox).
At the bottom of the list, turn CONVERSATIONS to off.