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alya
Staff
alya
Staff
June 2, 2025

Troubleshooting Tip: FortiMail SMTP error 530 5.7.5: 'Client was not authenticated to send anonymous mail'

  • June 2, 2025
  • 0 replies
  • 1037 views
Description

This article describes how to fix FortiMail SMTP error 530 5.7.5: 'Client was not authenticated to send anonymous mail'.

 

Log sample:

 

to=<userxx@example.com>, delay=00:00:09(tries=1), xdelay=00:00:05, mailer=esmtp, pri=138889, relay=smtp-mail.outlook.com [x.x.x.x], dsn=5.0.0, stat=Service unavailable(Reason from remote:530 5.7.57 Client not authenticated to send mail. [BN9PR03CA0329.namprd03.prod.outlook.com 2023-07-05T06:28:12.749Z 08DB7D1151B70CE7])
Scope FortiMail Cloud.
Solution

Common reasons include:

  • SMTP authentication is not configured or is misconfigured in FortiMail.
  • Incorrect credentials (username/password).
  • Sending from an IP or domain not allowed by the SMTP relay (e.g., not part of a permitted relay group in Microsoft 365).
  • TLS is required but not enforced.
  • Using a port that does not support anonymous or unauthenticated connections.

 

This KB article focuses on the common issue point 3:

Connector Configuration between FortiMail and Microsoft 365.

 

In Microsoft 365 Admin Center:

  • Go to Exchange Admin Center -> Mail Flow -> Connectors.
  • Create a new connector:
    • From: Partner organization.
    • To: Microsoft 365.
  • Add FortiMail's hostname to the allowed list.
  • Select the Reject email messages if they are not sent over TLS.

 

Note: For the error 530 5.7.57: 'Client was not authenticated to send anonymous mail', shown in FortiMail logs as below:

 

EXC1.png

 

This issue is resolved by enable anonymous SMTP option connections on the Exchange Receive Connector used by FortiMail, as shown below:

 

Screenshot 2026-04-01 141636.png
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