Troubleshooting Email Errors: Enabling IMAP and SMTP Access in Office 365

This article outlines troubleshooting steps for common errors and messages encountered during the setup and configuration of custom emails using IMAP and SMTP.

Log in to the Microsoft 365 admin portal at https://admin.microsoft.com/ as an administrator. Navigate to the Org settings screen and locate the Modern authentication entry:

• Enable ‘Turn on modern authentication’ for OAuth flows.
• Enable IMAP and SMTP for App passwords flows.

Next, proceed to the Users screen:

• Select a user and click on the Mail tab.
• Click on Manage email apps.

• Enable IMAP, POP, and authenticated SMTP to activate these protocols for the selected account.

Please note that changes may take 20-30 minutes to take effect.

Exchange administration
You can find the same mailbox/user settings through Exchange administration screens:

How to Resolve SMTP Authentication Error 535: 5.7.139 in Microsoft Login

The error 535: 5.7.139 Authentication unsuccessful, SMTP Client Authentication is disabled occurs when SMTP authentication is turned off for the account. Please refer to the steps below to resolve the issue:

User-Level SMTP Settings

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.

Global 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.

Additional Tips

• After updating the settings, retry the email configuration in your system.
• If the issue persists, ensure that your email client supports SMTP authentication and is configured correctly.