@RGJ I want to thank everyone for their help. I finally, finally have this resolved. The tweak for office365 is to use DISCOURSE_SMTP_AUTHENTICATION: login.
The offce365 smtp server and port config are smtp.office365.com:587 with STARTTLS enabled, which is the default anyway.
And the username is the full email address at the organization using office365, usually myaccount@mycompany.onmicrosoft.com. This may or may not be the same as your notification email.
Here is my final config:
DISCOURSE_SMTP_ADDRESS: smtp.office365.com
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: myacct@mycompany.onmicrosoft.com
DISCOURSE_SMTP_PASSWORD: mypassword
DISCOURSE_SMTP_ENABLE_START_TLS: true
DISCOURSE_SMTP_DOMAIN: outlook.com
DISCOURSE_NOTIFICATION_EMAIL: myacct@mycompany.com
DISCOURSE_SMTP_AUTHENTICATION: login
according to Mail Tester it is 10 for 10!