Microsoft 365 Error Codes

Fix SMTP OAuth Error 535 5.7.3 Authentication Unsuccessful in Microsoft 365

Natalie Taylor Oct 1, 2026 870 views

Question details

The user needs to resolve the '535 5.7.3 Authentication unsuccessful' error when trying to connect an application to smtp.office365.com using OAuth.

How to Fix SMTP OAuth Error 535 5.7.3 Authentication Unsuccessful

Product: Exchange Online / Microsoft 365
Device & OS: not provided
Scenario: Sending email through a third-party application using OAuth authentication with the smtp.office365.com server.
Observed behavior: The application fails to authenticate and returns SMTP Error 535 5.7.3, typically due to incorrect permissions, invalid token scopes, or disabled SMTP AUTH settings.

Before you start

Ensure you have Global Administrator or Exchange Administrator privileges in your Microsoft 365 tenant to modify user mailbox settings and Azure AD app registrations.

Solution 1Recommended

Enable Authenticated SMTP for the User Mailbox

This error frequently occurs because Authenticated SMTP is disabled for the specific user account in the Microsoft 365 Admin Center.

Microsoft disables basic authentication and SMTP AUTH by default in many tenants to improve security. You must explicitly enable it for the mailbox your application uses to send emails.

Access the Microsoft 365 Admin Center

Log in to the Microsoft 365 Admin Center using your administrator credentials and navigate to the 'Users' > 'Active users' section.

Open Mail Settings

Locate and click on the user account that the application is trying to authenticate with. In the properties pane that appears, click on the 'Mail' tab.

Manage Email Apps

Under the Email apps section, click on 'Manage email apps'. Ensure the 'Authenticated SMTP' checkbox is selected, then click 'Save changes'.

Enable Authenticated SMTP for the User Mailbox

Propagation Time: Changes to mailbox settings may take up to 30 minutes to propagate across Microsoft 365 servers before the application can successfully authenticate.

Solution 2

Configure Correct OAuth Token Scopes and Permissions

If you are using the client-credentials flow, the OAuth scope and Azure AD permissions must be correctly targeted at Exchange Online.

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Frequently Asked Questions

What causes the 535 5.7.3 Authentication unsuccessful error in Microsoft 365?

This error is triggered when an application fails to authenticate with smtp.office365.com. Common causes include disabled SMTP AUTH for the sending mailbox, incorrect OAuth token scopes, missing Exchange Online permissions, or tenant-wide security defaults blocking legacy authentication protocols.

Is SMTP OAuth the same as the Microsoft Graph API?

No, SMTP OAuth and Microsoft Graph API are separate mechanisms. SMTP OAuth is used to authenticate legacy protocols (IMAP/POP/SMTP), whereas Microsoft Graph is a modern REST API. If you are building a new application, Microsoft highly recommends using the Graph API for sending emails.

Why is Authenticated SMTP disabled by default for my users?

To protect against unauthorized access and modern cyber threats, Microsoft disables basic authentication and SMTP AUTH by default in new tenants. Administrators must explicitly enable SMTP AUTH only for specific mailboxes that genuinely require it for third-party application integrations.

Microsoft Office Error Codes