Troubleshooting Tip: FortiMail SMTP Error: '530 5.7.5 Client not authenticated to send mail'

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Description	This article describes how to fix FortiMail SMTP Error: '530 5.7.5 Client not authenticated to send 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.