Certificates accepted/rejected depending on what service I use...

Hey everyone. I was at my local star bucks, which has open free wifi. And I (against my better judgement) deceided to check my school email. I was using their wifi network. when I went to go login, I was presented with the safari prompt "Can not Verify Server identity". I clicked 'details' and apparently safari does not trust the *.njit.edu certificate. The signing authority is digicert.

Now what's odd is that if I switch to 3G. Using the 3G network, safari has absolutely no problem accepting it, and doesn't prompt me or anything. So what's the deal? I've done this multiple times, in multiple locations. Any idea's?

Could it be this...............

iOS 4.2 displays 'Cannot Verify Server Identity' notifications when connecting through a Microsoft Forefront TMG 2010 gateway