Why are marked E-Mails by iPhone's Mail.app not shown in GMail's marked folder? I have a GMail account (Google Apps with my own domain) hooked up as an exchange account in my iPhone.

If I mark an E-Mail (as shown in the screenshot) on my iPhone, it is not regonized by the GMail Web Client aka shown in the marked folder.

Do I have to configure something in my iPhone to fix this or is this not possible?

The text in the screenshot is german, but I think it is clear what means what. :-)
I dont think this is possible using the iPhone mail app. Marking an item with the iPhone mail app doesnt 'star' it on Google.

I'd recommend trying the official Gmail App for iPhone and iPad.