Strange IP issue -- Windows 2003

I apologize if this has been talked about. I've searched for this issue, but
it is hard to figure out exactly what to search for on this issue.

What we have is a Windows 2003 standard server that is part of a domain and
acts as a web server outside of our firewall. All of the domain machines are
inside the firewall. We allowed access from main IP of the server through
the firewall to needed ports on internal machines. But what we've seen
happen is that the server doesn't always use that main IP when it tries to
contact the internal machines. For example, synchronizing time to the main
DC, instead of using the main IP of 1.1.1.1, it will use one of the other
IP's that are on the server for one of the other websites, like 1.1.1.50.

The server is on a different subnet and VLAN than the internal machines, not
to mention that it is outside the firewall. As long as it uses its main IP,
we don't have issues. So I'm curious, is there a way to force it to always
use the main IP as the source IP when talking to this subnet (the internal
servers)?

Thank you,
-Ben