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According to Wertheimer, the email was sent to and from nondescript addresses that were later confirmed to belong to combatants. "It is surely the case that the sender and receiver attempted to avoid allied collection of this operational message by triggering presumed "spam" filters (PDF)." From a surveillance perspective, Wertheimer writes that this highlights the importance of filtering algorithms. Implementing them makes parsing huge amounts of data easier, but it also presents opportunities for someone with a secret to figure out what type of information is being tossed out and exploit the loophole.
By stripping out this flag, these ISPs prevent the email servers from successfully encrypting their conversation, and by default the servers will proceed to send email unencrypted. Some firewalls, including Cisco's PIX/ASA firewall do this in order to monitor for spam originating from within their network and prevent it from being sent. Unfortunately, this causes collateral damage: the sending server will proceed to transmit plaintext email over the public Internet, where it is subject to eavesdropping and interception.