I use a junk account, too for signups. Recently, I found an outfit called inboxgenius.com. All my incoming mail flows thru them and they filter out spam. Have been using them for about a month and it works great. Cheap, too.
I thought I had a particularly bad problem with spam. But I guess not, given this article. My stats from the filter company says they are filtering 80.17% of my email this month.
I keep thinking that there must be some way for legitimate senders to identify themselves to my account in such a way that if they fail to do so, the mail does not come through. Is it really that complicated?