I ran into this issue where SMTP would not send from within Outlook 2016 running on Win 10, tried every known configuration of SMTP; both encrypted and unencrypted with the same outcome.
I used the System File Checker by right clicking on the Start button and selecting Command Prompt (Admin). (You’ll need to click Yes to allow it to make changes to the computer.)
When the Command Prompt appears, type (or paste) sfc /scannow then hit Enter.
Restart the PC once the verification process has finished.
MystaJoneS 