Enable SMTP AUTH for specific mailboxes
Open the Microsoft 365 admin center and go to Users> Active Users.
Select the user and click Mail in the flyout that appears.
In the Email apps section, click Manage email apps.
Check the Authenticated SMTP setting: unchecked = disabled, enabled = enabled.
To set up Office 365 SMTP settings:
After signing in to the Outlook Mail app, use the toolbar and navigate to Settings> Mail> POP and IMAP. Then click on More Settings> Outgoing Mail Server to set up your SMTP settings. https://sites.google.com/view/office-365-smtp-settings/