Market Downloads, GTalk and GMail notifications fail when on company VPN I've noticed that when I login over WIFI to my company VPN the following problems occur:


GTalk does not login.
No GMail notifications received.
I can access the Android Market but when I attempt to download an application it fails.


All these work ok on my home WIFI and 3G connections.
Does anyone have any idea what would cause this?
What protocols/ports do these services use and is it possible to edit them?
I think GTalk uses port 5228, and the notifications and Market downloads both use the GTalk protocol internally.