Email Connector Error - 535 5.7.3 Authentication unsuccessful

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Hi all,   I can log in to my account successfully with my Basic Credentials on Email Connector module.   Though, the case we use the same account on our Company's Microsoft Entra connection we got "535 5.7.3 Authentication unsuccessful" error.   We have checked the permissions but we could not solve the problem.   Can you help us with this situation?
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It sounds like you're hitting the classic "Basic Auth vs. Modern Auth" wall. Since you can log in manually with credentials but fail through the Microsoft Entra (formerly Azure AD) connection, the issue usually isn't about what your password is, but how the server is being told to verify it.

The 535 5.7.3 Authentication unsuccessful error in a Microsoft 365 environment typically triggers when the system expects an OAuth2 token but receives standard credentials, or when security defaults are blocking the attempt.

Try the following 3 things:

1. Enable Authenticated SMTP

Even if the account has the right permissions, Microsoft often disables SMTP AUTH at the tenant or mailbox level by default for security.

  • Go to the Microsoft 365 Admin Center.
  • Navigate to Users > Active users and select the specific account.
  • Click the Mail tab and select Manage email apps.
  • Ensure Authenticated SMTP is checked.

2. Security Defaults & Conditional Access

If your company has Security Defaults turned on in Entra ID, Basic Authentication is automatically blocked.

  • The Conflict: Basic credentials cannot satisfy Multi-Factor Authentication (MFA).
  • The Fix: You likely need to register the Email Connector as an App Registration in Entra ID and use OAuth2 instead of "Basic" inside the module settings.

3. App-Specific Passwords

If your organization requires MFA and you must use Basic Authentication (not recommended), a standard password won't work.

  • You would need to generate an App Password from the user's Microsoft Security settings.
  • Note: This only works if your "Allow access to legacy authentication" policies permit it.


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