For DoD it's already a requirement that any emails with URLs or other "clickable links" are digitally signed (possibly this is true for attachments too, I'm not

For DoD it's already a requirement that any emails with URLs or other "clickable links" are digitally signed (possibly this is true for attachments too, I'm not sure). However this is a policy which is not computer-enforced, which means of course that it's fairly useless in practice.