Microsoft SMTP AUTH disabled: inspect the effective mailbox policy

Define the decision

Microsoft exposes both organization-wide and per-mailbox SMTP AUTH controls. A mailbox override can differ from the tenant setting. The error should be investigated by an administrator who can see the effective policy.

Work through the procedure

1. Capture the full SMTP authentication response.
2. Ask the administrator to inspect the organization setting.
3. Inspect the mailbox override and any applicable security policy.
4. Select an approved supported submission path, then retest the actual mailbox.

Worked example

The following is a synthetic example for this procedure, not a customer result or performance benchmark.

Read-only Exchange Online PowerShell checks:
Get-TransportConfig | Format-List SmtpClientAuthenticationDisabled
Get-CASMailbox -Identity [email protected] | Format-List SmtpClientAuthenticationDisabled
Mailbox value: true disables, false enables, blank inherits the organization setting

Read the result

These commands inspect settings; they do not change them. A successful browser login and a disabled SMTP protocol can both be true. Keep the policy finding with the connection error.

Check before moving on

1. Have an authorized administrator run tenant diagnostics.
2. Verify IMAP independently if incoming replies are required.

Limits and next action

Do not enable SMTP AUTH broadly as a troubleshooting shortcut. The administrator should evaluate the mailbox’s intended use and organizational security requirements.

Source: Microsoft: SMTP AUTH organization and mailbox settings