go to settings, email, account ... and choose the frequency that iphone will empty the trash can ... unfortunately you can not empty the whole thing at once ... you have to do it one message at a time ... or let the iphone do it on a daily basis ...
That is just not acceptable. I don't want that filth sitting in my iPhone for a whole day. There should be a way to select all mail in the trash at once and securely erase it. Hope someone at Apple is paying attention to this.