To hide the client's system IP address and hostname from the headers of emails sent through a Postfix mail server, you can use the smtpd_client_restrictions parameter in the main.cf configuration file.
The smtpd_client_restrictions parameter specifies restrictions that the Postfix server should apply to incoming SMTP connections from clients. By default, the server will include the client's IP address and hostname in the headers of emails sent through the server.
To hide the client's system IP address and hostname from the headers of emails sent through the server, add the permit_mynetworks restriction to the smtpd_client_restrictions parameter. This will allow the server to hide the client's system IP address and hostname if the client is connecting from a trusted network.
For example:
smtpd_client_restrictions = permit_mynetworks
It's important to note that this is just one example of how to hide the client's system IP address and hostname from the headers of emails sent through a Postfix mail server. There are many other parameters and settings that you can configure to customize the behavior of the server. Consult the Postfix documentation and online resources for more information on how to configure the smtpd_client_restrictions parameter.