Firewall in separate domain with one-way trust

ISA Server 2006 as domain controller of its own DMZ forest

One-way trust created, so the DMZ forest trusts the Enterprise forest accounts.

ISA Server 2006 authenticates requests at the ISA edge

Complex to configure

Scales well across an Enterprise solution.

For detailed instructions, see Using ISA Server 2004 with Exchange Server 2003 at this Microsoft Web site: http://go.microsoft.com/fwlink/?LinkId=109215.