Bayesian anti-spam filters count word-frequency in suspect messages and compare the results to profiles of word-frequency in spam and ham. Defeating this requires that your spam include a lot of natural human prose. So spammers have started to mine the Gutenberg Project and other sources of human-generated ASCII and dumping random hunks of literature into their messages to get around the filters.
Blogger and journalist Clive Thompson found an excerpt from Chapter 20 of The Master Key by Wizard of Oz author L Frank Baum in a message that had as its subject line “the big unit” (no prizes for guessing what the rest of it was hawking).