Not able to send email via zoho helpdesk

Not able to send email via zoho helpdesk

I have enabled TLS service on email yet it is showing same error.
Error while processing the request! javax.mail.MessagingException: 5.7.3 STARTTLS is required to send mail [MA0PR01CA0089.INDPRD01.PROD.OUTLOOK.COM 2023-07-17T11:24:37.605Z 08DB85D83ABE3E20]​