@Michael_Dahlgren_Nie
Probably I’ve found what do you need. You have to modify the file /etc/postfix/canonical by adding at the end of the file this line:
user@yourdomain.ext user@ispdomain.ext
Replace user and yourdomain/ispdomain with real values.
Then launch command postmap /etc/postfix/canonical and check that it doesn’t return errors.
If yes restart postfix with systemctl restart postfix
I don’t know if you can send emails with ISP domain without finish in SPAM: you can try. Remember to keep enabled smarthost configuration.
If it works you can set your NethServer as SMTP server into Outlook configuration.
Wait for feedback.