Im using Xeams on QNAP as home mail server to get e-mails from my ISP and send them through the mailserver of my ISP.

I have no local domain configured in Xeams.

I have added a ALLDOMAINS entry local host file

I have configured the smart host configration. 

But the at the postfix logs of the smarthost server I find:

01.11.2020, 19:15:33 info NOQUEUE: reject: RCPT from sending server[IP of sending server]: 554 5.7.1 <recipient email adress>: Relay access denied; from=<sender email address> to=<recipient email address> proto=ESMTP helo=<helo name of my Xeams server>
01.11.2020, 19:15:33 info Anonymous TLS connection established from sending server[IP of sending server] to ISP server: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)

I've replaced hostnames and IP addresses.

Did you specify a user id/password in Smart Host configuration? The message "Relay access denied" means the receiving end (your ISP) will not accept outbound email without authentication.

I solved the problem myself. I'veactivaded SSL but I haven't changed to port form 25 to 465.

In this configuration Xeams is a SMTPS Client and not a SMTP server using just TLS. Here was the misunderstanding issued.