SMTP server error: User is locked by your organization's security defaults policy

ERROR

SMTP server error: 5.7.57 Client not authenticated to send mail.Error: 535 5.7.139 Authentication unsuccessful, user is locked by your organization's security defaults policy.

Meaning:

Your Office 365 account ([e-mail]) is blocked from using SMTP AUTH due to Security Defaults or Conditional Access policies in your Microsoft 365 tenant.

Steps to Fix the Issue

Option 1: Disable Security Defaults (Tenant Level)

1. Log in to the Microsoft Entra (Azure AD) Portal: https://entra.microsoft.com/

2. Go to: Identity → Overview → Properties

3. Scroll to bottom, and look for: "Security defaults" → Set to "No"

4. Save changes.

This disables strict security defaults and allows SMTP login using username/password.

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Option 2: Enable SMTP AUTH for the Specific Mailbox

1. Go to: https://admin.microsoft.com

2. Navigate to: Users → Active Users → click [e-mail]

3. In the right pane, click Mail → Manage email apps

4. Ensure "Authenticated SMTP" is enabled

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REFERENCES

• https://learn.microsoft.com/en-us/answers/questions/681904/smtp-authenication-failure-5-7-57-and-5-7-139

• https://stackoverflow.com/questions/24947434/setting-up-phpmailer-with-office365-smtp