Sending mail on behalf other domain

I have a server serving running ucs for domain
My partner however has a second mailbox on it for sending mail on behalf of
There is a SPF record for my server so sending is completely legal.
To have the email address working, I had te create a local domain

The problem is now that I don’t know how to send email to because my server wants to handle this local.

Where am I going wrong? Thank you in advance.

Hi @BenNieuwhof,

what you are looking for is often called “mail split domain” and can be achieved through transport maps in Postfix. I recommend to use the forum search as this topic has been discussed quite a few times already.

Thank you Felix. It tok a few hours to find the exact way but the Search Terms helped me out.