Can anyone suggest a good way of reporting excessive spam false positives to Google?

My main email address is with 1&1 but I route it all via GMail. In the last few days, GMail has decided to blacklist the main 1&1 email server so any incoming email that's not specifically white-listed via my filters and contact list is getting lost in the spam folder.

Obviously I've reported this to 1&1, but I'd like to be able to report it to Google since it's fairly obviously an excessive setting in the Gmail algorithms. As ever it's quite hard to know where to report it. I've sent feedback via the GMail settings button but like all Google feedback buttons it's a bit of a black hole with no hope of a direct response.