According to an investigation that was conducted in 2015, the military left itself open to major security issues because of its email system that did not use STARTTLS. The technology is used to encrypt emails and makes it so that they cannot be intercepted by those who might be trying to plot something nefarious or even just curious lookie–loos.
Luckily, that will all change very soon, as the Defense Information Systems Agency (DISA), a branch of the Pentagon that oversees emails, has stated that it will start using STARTTLS sometime within the year. Currently, the military uses its own cloud-based email system called mail.mil, which has around 4.5 million users.
STARTTLS is not a new technology by any means, being introduced over a decade ago. It is a way to upgrade an insecure connection and making it secure using SSL/TLS. While this upgrade can't happen soon enough, don't expect it to happen overnight. The expected timeframe for it to be completed is set for sometime in July, 2018.