@ricardo Wow, that’s weird. Have you successfully reached that account via smtp with any other app? That would be my next step.
If you have gotten it working with another app, I can try to assist by trying to set mine it up with my o365 account and see what it takes.
Also, there is this Cannot send e-mails with SMTP using passbolt with OFFICE 365 which suggests that maybe o365 requires some headers or something that postfix sends by default.