FortiMail - SMTP Auth for One LDAP User

Good morning,

we need to enable an external application to send emails via SMTP through our FortiMail. The application is in the Azure cloud and wants to authenticate to the FortiMail.

Now the question is how I have to configure this with the authentication. Without authentication it already works if I create a new rule under Access Control => Receiving. But as soon as I activate the authentication there and the app tries to log in with the LDAP connections, the sending fails.

Connection failed (Login failed (Plain=The server rejects the request (503 5.3.3 AUTH not available
), Login=The server rejects the request (503 5.3.3 AUTH not available
))