When configuring a Microsoft 365 mailbox in BoldDesk using IMAP and SMTP, you may encounter authentication or connection errors if the required email protocols or authentication settings are disabled.
This article explains how to verify and enable Modern Authentication, IMAP, and SMTP Authentication in Microsoft 365. It also includes troubleshooting steps for resolving SMTP Authentication Error 535: 5.7.139.

Common Symptoms

You may encounter one or more of the following issues when configuring a Microsoft 365 mailbox in BoldDesk:

• Unable to connect a mailbox using IMAP or SMTP.
• Mailbox validation fails during configuration.
• OAuth authentication does not complete successfully.
• Email synchronization does not start.
• SMTP authentication errors occur.
• The following error message is displayed:
  • 535: 5.7.139 Authentication unsuccessful,
  • SMTP Client Authentication is disabled.

Resolution 1. Verify Modern Authentication Settings

Modern Authentication uses OAuth 2.0 and is required for secure authentication with Microsoft 365 services.

1. Sign in to the Microsoft 365 Admin Center.

2. Navigate to Settings > Org settings.

3. Locate and open Modern Authentication.
   

   
   
   
   

4. Verify the following settings:

   • Enable Turn on modern authentication for OAuth-based authentication.
   • Ensure the required authentication methods are allowed for your organization.
     
   
   
   

5. Save any changes.

Resolution 2. Verify IMAP and SMTP Settings for the Mailbox

If IMAP or SMTP Authentication is disabled for the mailbox, BoldDesk may be unable to validate or synchronize emails.

1. In the Microsoft 365 Admin Center, navigate to Users > Active Users.

2. Select the mailbox user.

3. Open the Mail tab.

4. Select Manage email apps.
   

   
   
   

5. Verify that the following options are enabled:

   • IMAP
   • Authenticated SMTP
     
   
   
   

Resolution 3. Verify Mailbox Settings in Exchange Admin Center

The same protocol settings can also be reviewed in the Exchange Admin Center.

1. Open the Exchange Admin Center.
   

   
   
   
   
   
   

2. Navigate to Recipients > Mailboxes
   

   
   
   

3. Select the affected mailbox.
   

   
   
   

4. Open the mailbox settings.
   

   
   
   

5. Review the email protocol configuration.

6. Confirm that:

   • IMAP is enabled.
   • Authenticated SMTP is enabled.
     
   
   
   

Resolution 4. Resolve SMTP Authentication Error 535: 5.7.139

Cause
This error occurs when SMTP Authentication is disabled either:

• For the mailbox.
• For the Microsoft 365 tenant.

Verify User-Level SMTP Authentication

1. Log in to the Microsoft 365 Admin Center with administrator credentials.
2. Navigate to Users > Active users.
3. Select the user account you are trying to configure.
4. In the user’s profile, click Mail.
5. Under Email apps, click Manage email apps.
6. Locate Authenticated SMTP:
   • If it is checked, SMTP authentication is enabled.
   • If it is unchecked, enable it by checking the box.

Verify Tenant-Level SMTP AUTH Settings

1. Access the Exchange Admin Center.
2. Navigate to Settings > Mail flow.
3. Select SMTP AUTH.
4. Ensure the setting Turn off SMTP AUTH is not enabled globally for your tenant.

Frequently Asked Questions

1. Why does mailbox validation fail even when the credentials are correct?
   Mailbox validation can fail if IMAP, SMTP Authentication, or Modern Authentication is disabled for the mailbox or tenant.

2. How long do Microsoft 365 email setting changes take to apply?
   Most changes take effect within 20 to 30 minutes, although propagation time may vary.

3. What causes SMTP Error 535: 5.7.139?
   This error occurs when SMTP Authentication is disabled either at the mailbox level or tenant level.

4. Is Modern Authentication required for Microsoft 365 OAuth connections?
   Yes. OAuth-based authentication relies on Modern Authentication being enabled in Microsoft 365.