MPX200 Smartphone and INTERNET_31 error using Activesync 3.7.1 internet passthrough (error)

I've come across a frustrating issue and would greatly appreciate if
someone could help me resolve.

1.) am able to successfully synch my smartphone to our
outlook.company.com web mail server wirelessly from my MPX200
smartphone (via activesync on phone) via internet

2.) am able to successfully synch my MPX200 smartphone to our
outlook.company.com web mail server on a PC running activesync 3.7.1
using USB cradle and activesync internet pass through option in server
rules section

3.) am able to successfully synch when same PC as above is connected
to my work network via VPN

** 4.) when I connect this PC to our corporate network, am unable to
synch my mpx200 smartphone via activesync using internet passthrough
as Activesync on smartphone reports an INTERNET_31 error (as if it
cannot access the internet)

This has been a struggle and would appreciate any help that can be
provided.