I have had this warning become a recurring issue since I got back from deployment in July, but I've also noticed that it still does actually sync and retrieve my email. I honestly believe that it is an issue on AKO's end and not the phone itself.
I will say to double check your settings and ensure you can log in with the password you have on a PC, after you successfully logged in, double check your password settings on your phone to ensure they're the same, and make sure you have the following settings:
Incoming server:
Server: imap.us.army.mil
Username: user.name
Password: password
Advanced Settings
Port: 993
Use secure connection checked
Use secure password (TLS): Auto
Verify Certificate unchecked
Outgoing Server:
SMTP Server: mailrouter.us.army.mil
Username: user.name
Password: password
Advanced Settings
Port: 465
Use secure connection checked
Use secure password (TLS): Auto
Authentication: Auto
Verify Certificate unchecked
Just ensure that your settings also match those. Since the error message you're getting is with mailrouter (like mine), there should only technically be an issue with sending emails and not receiving them, although I believe I've been able to send emails as well, but I still do get the warning. It's just as bothersome to me to see, but as I stated before, I believe the issue is with AKO and not your phone.