VB.NET: ‘535: 5.7.3 Authentication unsuccessful’ error when trying to send mails using OAuth from an Outlook free account

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Summary

The ‘535: 5.7.3 Authentication unsuccessful’ error occurs when attempting to send emails using OAuth from a free Outlook account in a VB.NET project. This issue arises due to misconfiguration in the Azure Portal or insufficient permissions.

Root Cause

The root cause of this error can be attributed to the following:

  • Incorrect Azure AD configuration: The Azure AD configuration, including the authority, client ID, tenant ID, and redirect URI, may not be set up correctly.
  • Insufficient permissions: The application may not have the necessary permissions, such as ‘SMTP.Send’, to send emails using OAuth.
  • Security defaults: The security defaults on the tenant properties may be enabled, which can prevent authentication.

Why This Happens in Real Systems

This issue occurs in real systems due to:

  • Lack of understanding of OAuth: Developers may not fully understand the OAuth flow and the required permissions.
  • Misconfiguration of Azure AD: The Azure AD configuration can be complex, and misconfiguration can lead to authentication issues.
  • Insufficient testing: The application may not be thoroughly tested with different scenarios, leading to unexpected errors.

Real-World Impact

The impact of this issue can be significant, including:

  • Failed email delivery: The application may not be able to send emails, leading to failed delivery and potential business losses.
  • User frustration: Users may experience frustration and disappointment when they are unable to send emails.
  • Reputation damage: The application’s reputation may be damaged due to its inability to perform a critical function.

Example or Code

Dim oauth2 As SaslMechanismOAuth2
oauth2 = New SaslMechanismOAuth2(Str_From, Str_AccessToken)
Await smtp.AuthenticateAsync(oauth2).ConfigureAwait(False)

This code snippet demonstrates the authentication process using OAuth.

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Verifying Azure AD configuration: Ensuring that the Azure AD configuration is correct and up-to-date.
  • Checking permissions: Verifying that the application has the necessary permissions to send emails using OAuth.
  • Testing thoroughly: Thoroughly testing the application with different scenarios to ensure that it works as expected.

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience: Limited experience with OAuth and Azure AD configuration.
  • Insufficient knowledge: Limited understanding of the required permissions and security defaults.
  • Inadequate testing: Inadequate testing of the application, leading to unexpected errors.

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