OAuth for Outgoing Mail - Unknown host: smtp

Attempting to set up the newly supported OAuth for Outgoing email (O365) and keep getting an error, "Unknown host: smtp."

We've been using OAuth/O365 for a few years now for incoming email and we have no problems. Also, when setting up things for Outgoing mail, Authorization is successful. So the credentials are correct. I get the error only when trying to save, and then again when attempting to send mail. INCOMING mail, using OAuth/365 with the same account, works perfectly fine. Therefore, and since this is a new feature, I suspect there's a config file somewhere with the wrong server address for O365. Maybe.