After having some problems with MSIE i activated on one of my sites the config.absRefPrefix directive and set a prefix to all of my URL. I knew that this might cause some problems, especially with RealURL.
But after some testing i found out that RealURL is working as expected but Powermail forms doesn’t. They submit the form to a wrong URL. After some digging i found the solution. It seems that older versions of Powermail (mine was 1.4.7) doesn’t handle correctly the config.absRefPrefix setting.
So i upgraded to the latest version and this solved my problem. Maybe it will help some of you too.