How is spam sent?

Spam is sent either from:

1. Individual computers that have been infected with a virus / trojan - they connect to the Internet and download lists of email addresses and start sending out spam.

2. Misconfigured email servers (open relay) - some people setup or reconfigure mail servers incorrectly and receive mail from anyone and then redeliver it - spammers love these as servers are usually on high-speed Internet connections so can send more spam quicker!

3. Some ISP's are spammer-friendly - they are willing to take payment to setup servers and even offer to change IP addresses when those IP's get blacklisted.

4. Spammers may buy mail server services from ISP's using stolen credit card details.