Mail that contains or has been tainted by a noxious substance, especially the anthrax virus.
"Waxman and the committee's chairman, Rep. Dan Burton (R-Ind.) sent an unusual bipartisan letter late Tuesday to FBI Director Robert Mueller and Postmaster General John E. Potter, criticizing the delays in examining the mail and demanding answers. ...
'We urge you to work together to expedite the testing of collateral mail to answer critical questions about cross-contamination,' the two congressmen wrote."
Josh Meyer, Eric Lichtblau, "Anthrax Probe Is Assailed," The Los Angeles Times, November 1, 2001
"Greg Poland, a professor of medicine and infectious diseases at the Mayo Clinic in Rochester, Minn., said that although 'no envelope is airtight' and any powder inside could be squeezed out in processing, the probability of a significant number of anthrax spores escaping is low. ... So far, he said, no anthrax cases have been caused by what he called 'collateral mail.'"
Neely Tucker and Avram Goldstein, "Anthrax Threat Takes a Wider Scope," The Washington Post, October 24, 2001
Today's euphemistic phrase is itself based on another euphemism, the notorious collateral damage, a term that military double-spokespersons use to refer to the unintentional harm to civilian life or property that occurs during a military operation. The source of the euphemism is the word collateral, which in this sense means "additional but secondary" (collateral comes from the Latin collateralis, literally "side by side with").
Given this, collateral mail doesn't quite work because the inherent harm in such mail is inferred only via the phrase's relation to collateral damage. Of course, attempting to undergird militaryspeak with a rational etymological support is almost always a foolish exercise.