That’s what we in the cybersec business call an “oopsie daisy I made a little fucky-wucky”.
For real though, this isn’t a problem yet. The TL;DR is that Mali has a top-level domain “.ml”. Just like “.co.uk” for the UK. And the military uses the domain “.mil”. So lots of emails accidentally get sent to “[Military email]@[Military email server].ml” instead of sending to .mil.
So a bad actor could simply set up an e-mail server with .ml domains that mirror the military’s .mil ones, and start collecting all of those mis-addressed emails.
So why isn’t it an issue yet? Because we had a contract with Mali to manage their domain. They literally signed administrative rights for the .ml domain over. So the US was able to basically set up their own .ml mirrored sites, to capture all of those mis-addressed emails. They have captured thousands throughout the years, because military members keep misaddressing their emails. Supposedly containing all kinds of sensitive data. Everything from medical records to troop movements and equipment inspection reports.
But that contract ends this week, so Mali could 100% start registering their own domains when that contract expires and domain registrations begin expiring.
One solution to this would be to set the .mil mail servers to either correct or bounce all .ml addressed mail, no? It makes emailing legitimate .ml addresses more difficult, but requiring a second, dedicated gateway or mailserver for .ml would be at most inconvenient.
Making a typo that can send to the wrong place is a common error by anyone. Net security that allows it, presumably some of them from military intranets or in various correspondences without flagging it a problem, that's a huge mistake. The solution was a patchwork to make the problem a future one, which is so government typical. Probably would have required some reprogramming in COBOL, and they couldn't find anyone.
Given it's the military you would hope nothing actually serious was sent via email in the first place at least on a system connected to the internet. Yes personal records etc are important but they're rarely if ever national secrets.