gmaill / RBL: SORBS: sender is open HTTP proxy server

Hi Folks,
running plesk with a default nginx / apache config I notice the following information in email heraders

RBL: SORBS: sender is open HTTP proxy server

I use the default nginx / apache config done by plesk... now I'm a bit afraid of having trouble with this machine...

Could anybody plz advise how to check if I really run an open http proxy and second, how do I close this on my machine?

thanx in advance

Mat