DNS question, outgoing traffic identification

Lexington, KY
Here is a scenario: Let's say I have a domain z.com I have two hosts: a Windows 2003 R2 server that sitw behind a Debian Linux server which is connected to the ISP. Because the web site and mail are hosted outside the LAN here is how the addresses resolve: lan.z.com resolves to my Linux host. z.com and mail.z.com resolve to my mail/web provider's host. Now the problem: Outgoing connections from the Windows server are identified by the third party firewall as coming from mail.z.com whereas they are really coming from lan.z.com Nobody cares, until that firewall is set up to perform reverse DNS lookup. In case this helps, my Linux server hosted the mail and web site until these services were moved out to the third party hosting company. I need connections from the Windows server to be identified as coming from lan.z.com Where do I fix this? One of my hosts? The ISP? I doubt the mail/web host is the problem. Thanks!