Enable SMTP Client Authentication at the Tenant Level
    Log in to the [Microsoft 365 Admin Center](https://admin.microsoft.com/).
    Navigate to Settings > Org settings.
    Under Services, select Modern authentication.
    Ensure that the checkbox for Allow clients to connect using modern authentication protocols (IMAP, SMTP, etc.) is enabled.

Enable SMTP Authentication for the User If SMTP is disabled for the specific user account, you need to enable it:
    Go to the Microsoft 365 Admin Center.
    Navigate to Users > Active users.
    Select the user in question and open Mail settings.
    Under Email apps, ensure Authenticated SMTP is enabled.

Verify Multi-Factor Authentication (MFA) Settings If your user has MFA enabled, ensure that you're using OAuth2 for SMTP. Basic authentication won't work with MFA.

Check Security Defaults If Security Defaults are enabled in Azure AD, they might block SMTP authentication. To disable security defaults:
    Go to the Azure Portal.
    Navigate to Azure Active Directory > Properties.
    Scroll down to Manage Security Defaults and disable them.