Office 365 SMTP with TLS issues

Office 365 SMTP with TLS issues

When setting the SMTP server on port 587 (TLS) we get an error in Zoho desk. When we use 25 (PLAIN-TEXT) it works, but we cannot send emails.

The error message we see in 'Failed Email Alerts' is:

Error while processing the request! javax.mail.MessagingException: 5.7.3 STARTTLS is required to send mail [AM0PR10CA0100.EURPRD10.PROD.OUTLOOK.COM]

How can we fix this?