Using Classic Outlook on Windows for personal email, calendar, and contact management
I have had the same problems, too. It appears on my machine that McAfee "auto-enabled" output email scanning on 6 March 2019. You can check your Sent Items folder to see when the scanning started on your computer. I've disabled the McAfee Outlook Add In which then enables the McAfee Anti-Spam tab, from which you can turn off the output email scanning. I haven't seen McAfee owning up to any of this.