Thank you for posting this suggestion. Currently, only the Mailbox IMAP Connector has the ability to mark a message as read, possibly you can use this. We will make an effort to add this also to the Outlook 365 Connector, but it is not planned for the near-term.
Hi, this is revisted after a year and still not planned, so we will set to archived. If this idea is still relevant please resubmit it in the ideas forum.