Exchange Connection Problems

How to Enable SMTP AUTH for a Single Microsoft 365 User

Maira Mehtab Sep 21, 2026 868 views

Question details

An administrator wants to enable SMTP AUTH for one specific Microsoft 365 mailbox without requiring all users to use it or disabling modern authentication for the whole organization.

Product: Microsoft 365
Device & OS: not provided
Scenario: Configuring email application access for a specific user requiring legacy SMTP authentication.
Observed behavior: The goal is to successfully allow SMTP AUTH for an individual user while maintaining overarching tenant-wide security policies.

Before you start

Ensure you have Microsoft 365 administrator privileges and verify whether your organization's tenant-wide security defaults or Conditional Access policies might override individual mailbox settings.

Solution 1Recommended

Enable Authenticated SMTP via Microsoft 365 Admin Center

Use the Microsoft 365 Admin Center interface to quickly toggle Authenticated SMTP settings for a specific active user.

This method allows you to enable SMTP AUTH on a per-mailbox basis. Keep in mind that OAuth (Modern Authentication) is preferred for better security, and basic authentication is subject to deprecation. Microsoft 365 tenant-level security defaults may still block SMTP AUTH even if this is enabled for the individual mailbox.

Access Active Users

Select the User

Locate and click on the specific user for whom you want to enable SMTP AUTH to open their properties pane.

Manage Email Apps

Click on the 'Mail' tab in the user properties panel, then select 'Manage email apps'.

Enable Authenticated SMTP

Check the box next to 'Authenticated SMTP' to enable the feature, and click 'Save changes' to apply the new settings.

Policy Conflicts: If your organization has Security Defaults enabled or an overarching authentication policy blocking basic authentication, it may override this per-user setting and still block SMTP AUTH.

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Frequently Asked Questions

Why is SMTP AUTH still failing after I enabled it for the individual user?

If tenant-wide security defaults or Conditional Access policies are enabled, they will block basic authentication (which SMTP AUTH relies on), overriding any individual mailbox settings.

Can I enable SMTP AUTH for a single user using PowerShell?

Yes. You can connect to Exchange Online PowerShell and run the command 'Set-CASMailbox -Identity user@domain.com -SmtpClientAuthenticationDisabled $false'.

Is it safe to leave SMTP AUTH enabled?

Basic authentication is less secure than Modern Authentication (OAuth) and is vulnerable to brute-force attacks. Microsoft recommends transitioning to applications that support OAuth and disabling SMTP AUTH whenever possible.

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