Do terrorists use spam to shroud their secrets? by Paul Ducklin.
Paul reviews a paper by Michael Wertheimer (NSA) about subversion of the random number generator Dual_EC_DRBG that mentions as an aside, terrorists using spam subject lines to escape scrutiny of their email. Paul makes the point that discarding spam may lead to discarding of the intelligence you are seeking. Good read.
Spam filtering seems like a low-lying fruit way to avoid scrutiny.
One assumption for new security thinking should be: All your communications are being intercepted.
The intercept assumption is necessary in light of highly probable interception of all digital traffic by the NSA. That assumption also prevents solutions based on optimism concerning the data available to the NSA and others.
How would you use spam to keep private things private?