Change the headers in postfix

Hi everybody,

I need help to configure my vps. I use postfix software to send e-mail and when I send e-mail from my server using PHP, I can see the local host in the mail. 39; header that displays as follows:

Code:

Received: from localhost (gateway1.mydomain.com [104.xxx.xxx.xx]) by mail.mydomain.com (Postfix) with the ESMTPSA ID E118F7B89DCD for ; Mon, 26 Nov 2018 14:49:35 -0500 (EST)

I want to delete and replace it with my domain name so that my e-mail address appears as genuine.

Do you know how I can delete the local host to replace it with my domain name?

Any advice would be very appreciated.

Thanks in advance.