Exchange Connection Problems

How to Fix WP Mail SMTP Error 535 5.7.139 Authentication Unsuccessful

Chanuka Geekiyanage Oct 9, 2026 869 views

Question details

The user needs to resolve an SMTP authentication failure when connecting a Microsoft 365 account to WP Mail SMTP.

How to Fix WP Mail SMTP Error 535 5.7.139 Authentication Unsuccessful

Product: Microsoft 365 / WP Mail SMTP
Device & OS: not provided
Scenario: Configuring WordPress to send emails using a Microsoft 365 account via WP Mail SMTP plugin.
Observed behavior: The connection fails with Error 535 5.7.139 Authentication Unsuccessful, even after configuring App passwords, MFA, and disabling default security settings.

Before you start

Ensure you are logged into the Microsoft 365 Admin Center with a different Global Administrator account, as you will need to modify the permissions of the affected SMTP account.

Solution 1Recommended

Remove Administrator Privileges from the SMTP Account

Microsoft 365 enforces strict security policies on administrator accounts that block basic SMTP authentication. Demoting the account to a standard user removes these restrictions.

Accounts with global or specialized admin roles in Microsoft 365 are subject to elevated security defaults. Standard troubleshooting steps like generating App Passwords or toggling MFA will not override these account-level security blocks for SMTP.

Access Active Users

Select the Affected Account

Find and click on the user account that you are using for the WP Mail SMTP connection to open its properties panel.

Manage Roles

In the account panel, go to the 'Account' tab. Under the 'Roles' section, click on 'Manage roles'.

Remove Admin Access

Select the 'User (no admin center access)' option to strip administrator privileges from this account, then click 'Save changes'.

Wait and Retest

Wait for the permission changes to fully propagate across Microsoft's servers, then go back to your WordPress dashboard and send a test email via WP Mail SMTP.

Remove Administrator Privileges from the SMTP Account

Propagation Time: Role modifications in Microsoft 365 do not take effect instantly. It typically takes about 2 hours for the changes to fully propagate and for SMTP authentication to start working.

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Frequently Asked Questions

Why does Microsoft 365 block SMTP authentication for admin accounts?

Microsoft implements strict security defaults to protect highly privileged administrator accounts from unauthorized access. These security policies automatically block legacy protocols like basic SMTP authentication, regardless of other settings.

Will an App Password fix Error 535 5.7.139?

If the account has administrator privileges, an App Password will not bypass the error. The account-level security restriction must be resolved first by changing the user role to 'User (no admin center access)'.

How long does it take for Microsoft 365 role changes to apply?

After modifying user roles in the Microsoft 365 Admin Center, it can take up to two hours for the changes to fully sync across all Exchange servers. SMTP authentication will fail until this sync is complete.

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