Several years ago we moved all email services to Office356 and used a hybrid Exchange server to do so. After the migration we kept the hybrid Exchange server as a method for MPFs and other on-premise services to send email to Office365. We have not used it for any O365 management.
The issue we have now is all of our Outlook desktop clients are recently complaining about the certificate on the hybrid Exchange server, but really they do not connect it and are pointed to O365/Exchange Online. They never used to do this but something has changed and some Outlook clients have a pop-up box requiring the user to accept and continue.
Is there a way to either prevent our Outlook desktop clients from trying to connect to the hybrid Exchange avoiding the certificate pop-up, a change in DNS perhaps, or a way to make the hybrid so that we can still use it for on-premises SMTP services but otherwise nothing interacts with it?