The ISA firewall is configured to permit Internet users to access their e-mail to back-end exchange server. Internal users can access back-end exchange by OWA. Internet users cannot access their mailboxes by OWA.
Solution:
Configure the default gateway of Front-end network adapter to 192.168.1.254.
configure a static route in Front-end server
Route -p add 172.16.1.0 mask 255.255.0.0 172.16.1.253
