An amazing study conducted by Barracuda Networks showed that 95% of all email sent in 2007 was spam. This is up from 70% in 2004. It seems that as one side of the spam war attempts to make more clever spam filters, the spammers use technology to go around those filters. These figures are truly amazing. What amazes me more is that ANYONE would buy ANY product or service they learned about via spam. Someone out there must be buying or spamming would not be profitable.
Email services vary in their ability to weed out the spam. In my personal experience, Google does a much better job than Yahoo of getting rid of the junk and NOT deleting the emails I want to receive.
Read more about the spam study in this cnet article.