According to a prediction by Spamhaus, global spam reach 95% of all email traffic by the end of 2006.
Spam currently accounts for 75% of all email traffic arriving at most ISPs mail servers. Spam typically originates from two sources, either sent directly by the spammer, or sent by the spammer through a hijacked computer (proxy). Using either method it's easy for anti-spam systems to deal with.
Unfortunately over the last few months the source of spam has been changing. More spam is coming directly from the major mail relays of other ISPs. AOL is reporting that 90% of spam that hits its servers comes directly from other ISP mail relays.
Spammers have been releasing variations of stealth proxy spam software ("spamware") to take control of private computers, usually those on the world's broadband networks, and to use them to send out spam for pornography or illegal drugs from without the PC owner's knowledge or permission.
Spamhaus is currently advising mail services to take protective measures to avoid or lessen the problem. Throttling the outgoing mail from IPs of broadband customers, Separate the incoming and outgoing SMTP servers, Mandate email authentication (SMTP-AUTH) for all customers are all different ways that can solve various spam proxy problems according to Spamhaus.