I have been setting up an SMTP relay for an Office365 server, and eventually found the following combination to work. The setup on the O365 side was a whitelisted domain.
DISCOURSE_SMTP_ADDRESS: <yourdomainkey>.mail.protection.outlook.com
DISCOURSE_SMTP_PORT: 25
DISCOURSE_SMTP_DOMAIN: <yourdomain>
DISCOURSE_SMTP_ENABLE_STARTTLS_AUTO: true
Hope that helps.