- enable auto-forward of all mails to the gmailaddress
- disable MFA (not really a good idea) on the account
- enable SMTP auth on the user profile (https://admin.microsoft.com/ > active users > user > email > apps >verified SMTP)
- enable SMTP auth on the organisation (https://admin.exchange.microsoft.com/#/settings > mail flow > SMTP AUTH)
- edit Anti-Spam outbound policy and enable forwarding
- setup DKIM