The target host requires authentification (username/ password) through TLS.
So you have to check the postfix (?) configuration.
See here: https://blog.bartlweb.net/2011/02/smtp-relay-host-mit-authentifizierung-in-postfix-konfigurieren/
and
https://help.univention.com/t/eine-empfangerdomain-uber-relay-mit-auth/8320/12