To provide more info:
I've assigned a dedicated IP address to userdomain.com which is hosted under a reseller account and mail.userdomain.com.
However, when I send email from Outlook via SMTP mail.userdomain.com the headers are:
Received: from server.host.com (differentresellerdomain. [xxx.xxx.xx.xx])
by mx.google.com with ESMTPS id sadsadadgasdaserqz.254.2012.07.18.02.40.58
Wed, 18 Jul 2012 02:41:00 -0700 (PDT)
I have no idea why it is using a differentresellerdomain and its assigned IP address.
However, I want the "Received" header to be something like:
Received: from server.host.com (userdomain.com. [xxx.xxx.xx.xx])
Why am I trying to do this? Because the main server host IP got blacklisted. It should be de-listed shortly, but I'm trying to get around it for a hosted client by assigning them their own IP address to send through for their SMTP host.