This one is strange. My Outlook client will not connect to my Exchange system through Office 365 at my home network (only). After quite a bit of time diagnosing network issues, I disabled my Avast system. Low and behold, it works.
Even if I use the hotspot on my phone, Outlook connects just fine. Outlook for web connects just fine over any network with Avast on.
So it is just one network. I cleared all of the ports in the mail settings just to see what would happen, no effect. Also my Sonos controller doesn't work with Avast turned on. All of this started Monday, I'm assuming over an update. I did update and reinstall all of my network drivers, and of course reset the heck out of my routers, etc. The only thing that worked is shutting off Avast (I have the paid version).
Any idea as to why this may have started? I'm on Windows 10.