You might have just noticed this: as of mid-July 2021, there's no longer any MX record for the us.army.mil email domain. There's no A record, either, so any attempts to mail users at this domain will now bounce.
This appears to be planned, though its final shutdown date was perhaps delayed. Friends and industry colleagues pointed out that the US Army announced that they were going to shut down the AKO (Army Knowledge Online) email portal way back in 2015. From Army set to shut down AKO email:
Army Knowledge Online will soon be out of the email loop, as the Army will shut down AKO’s classified and unclassified email systems effective March 31, while users continue to move to DOD Enterprise Email (DEE) or other addresses.
AKO will continue to forward email to DEE or legacy accounts until June 30 (2015), after which any mail sent AKO’s email@example.com accounts will be returned to the sender, the Army’s Program Executive Office for Enterprise Information Systems said in an announcement.
Here we are, about six years later, but indeed that service does seem to have been shut off and all attempted email delivery attempts are now bouncing back, as promised.
Affected members of the military were told to refer to this AKO Email Shutdown Quick Reference Guide for more information.
For email senders, the best you can do here is hope that subscribers provided an alternate email address, assuming they wanted to continue to receive your email messages.
Thanks to Zach Rose, Jon Marburger, Kris Dougherty, Marc Powell and others who helped provide detail around this issue.