Complex sentence: Spammers collect email addresses from chatrooms, websites, customer lists, newsgroups, and viruses which harvest users' address books, these harvested email addresses are sometimes also sold to other spammers.
Split into simpler sentences: Spammers collect email addresses from chatrooms, websites, customer lists, newsgroups, and viruses that harvest users' address books. These collected email addresses are sometimes also sold to other spammers.