So i managed to set up postfix with authentication that rewrites from address to the required one, and i did it via shell instead of the webbgui (etc etc).
Pickle is that if i upgrade it the authentication stops working and i quite cant figure out why.
From log
Status=bounced ... said: 530 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM (in reply to MAIL FROM command))
It was quite easy in the end:
For whatever reason the [mailserver]:port part in /usr/local/etc/postfix/smtp_auth disappeared in the update.
So change it back to:
[mailserver]:port username:password
Voila it works!