I would double check a few things.
The SMTP Outgoing server is typically outlook.office365.com on port 587 for a Microsoft client.
In the Azure Portal app registration, check the API Permissions. It should not only have Graph delegated SMTP.Send, but it must be granted consent by a global admin (status green).
In Exchange Admin Center, the mailbox must have SMTP permission checked by the exchange admin. In rare cases, disabling this permission and enabling it again will enable the permission (ms, go figure).
If the configuration is correct according to the guide and it still isn't working, then check error logs on the server, and Admin Dashboard System Log in osTicket. Post any related errors that you haven't resolved.
