The trouble is, these filters are dumb: they can't tell the difference between a sexual solicitation sent by e-mail and a news story about restrictions on online pornography or between a computer virus and a story about a computer virus.

Compressed Data: Law Newsletter Has to Sneak Past Filters
There is nothing wrong with David Carney's spell-checker. It is on purpose that in his e-mail newsletter, Tech Law Journal, he misspells words like sex (sez) and pornography (pormography) and camouflages the names of computer viruses. If he did not, he explained last week in an editor's note, his journal would never get past the computers at readers' offices that screen incoming e-mail messages for references to sex or network security.
http://www.nytimes.com/2001/04/02/technology/02FILT.html?pagewanted=all