Just come across this Firefox plugin called "SiteAdvisor". I had install it and run it. It is so impressive on the way it integrate with Firefox, Google, and Yahoo. It will put an icon on each search result to indicate the safety of the website. If you do not reach the site from search engine, it will have a icon at the bottom right of your browser to give the same indication as well.
It is based on the database at server site so we do not need to update its database as what we do with our anti-virus software but yet we are keep to be most up to date.
Of course, it detect the threat more than just spam mail, here is what I had copy from their website:
SiteAdvisor helps protect you from all kinds of Web-based security threats including spyware, adware, spam, viruses, browser-based attacks, phishing, online fraud and identity theft.
And this software is free!!!! Just install it and feel it yourself. If you do not have a Firefox, just download it from the Firefox banner at the left banner of this blog.