Client Email Received From Header IP

[MorrisC]

On our server, we disabled mail function on php. Our clients are sending mails from their website through SMTP sendmail

here is the problem

Received: from localhost ([plesk-default-ip])
by smtp.gmail.com with ESMTPSA id w9sm2123415pbt.29.2015.09.11.14.23.42
for

on that localhost ip, it is showing my plesk default ip instead of my client's website ip....any way to make it to use my client's website ip instead?

NGINX is installed with php-fpm support for nginx

Thanks for all your help!