so far, we’ve given up and reconfigured to O365 and it suddenly worked… we’re at the point of making a few tweaks and not being sure what made it work… nevertheless - Office 365 config works with the following:
DISCOURSE_SMTP_ADDRESS: 'smtp.office365.com'
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: 'your.email@domain.com'
DISCOURSE_SMTP_PASSWORD: 'password'
DISCOURSE_SMTP_AUTHENTICATION: login
DISCOURSE_SMTP_ENABLE_START_TLS: true