Those clever terrorists hit upon a smart way of making their emails seem innocuous -- using spam-like subject headers so they looked like the sort of clutter that ought to be instantly deleted and ignored.
According to a reports by a Director of Research at the NSA [PDF], emails were uncovered from seized Taliban laptops abandoned by forces in Afghanistan. Michael Wertheimer explained: "The 'to' and 'from' addresses were nondescript (later confirmed to be combatants) and the subject line read: CONSOLIDATE YOUR DEBT. It is surely the case that the sender and receiver attempted to avoid allied collection of this operational message by triggering presumed 'spam' filters." [Quartz]