The wonderful people over at the Google Operating system wrote a post and put up a fun little explanation video of what Gmail does with spam.
I have to admit, ass far as spam goes gmail has to be the best service I’ve seen. Filtering the bad stuff and allowing the good stuff.
Also, I’m one of the good people who reports span should it ever get into my inbox. (after I post it, along with the email address it came from on the blog).
According to the Google Operating System:
"Many Google teams provide pieces of the spam-protection puzzle, from distributed computing to language detection. For example, we use optical character recognition (OCR) developed by the Google Book Search team to protect Gmail users from image spam. And machine-learning algorithms developed to merge and rank large sets of Google search results allow us to combine hundreds of factors to classify spam," explains Google. "Gmail supports multiple authentication systems, including SPF (Sender Policy Framework), DomainKeys, and DKIM (DomainKeys Identified Mail), so we can be more certain that your mail is from who it says it’s from. Also, unlike many other providers that automatically let through all mail from certain senders, making it possible for their messages to bypass spam filters, Gmail puts all senders through the same rigorous checks."
See the video below and read more here