I’m assuming this doesn’t involve intra-military emails, because that would be trivial to prevent. It’s probably because of people sending from another domain. Like if [email protected] is sending an email to [email protected], but he mistypes the .mil part because he is using his iPhone while riding his motorcycle with a girl on the back.
A more realistic example would be [email protected] sending an email to [email protected] to discuss some upcoming meeting about a new aircraft contract.