Why are some email senders unable to send email to "army.mil" email addresses?

Question

Our company is using a Microsoft 365 enterprise email. Some of our users are having trouble sending email to people with an "army.mil" email address. There are no NDR reports generated and message tracing has so far proven to be less than helpful as a result. We have tried sending emails with delivery and read receipts so I can narrow the focus of our troubleshooting but all emails from our company are successfully delivered. However, most users are unable to reach the destination mailbox and never receive a read receipt.

Answer 1

If you can send messages to other domains and only have problems with army.mil, the most likely reason is that army.mil has some policy that block the messages sent by your domain.

1. Firstly you could check if your domain has valid SPF record, valid DKIM domain keys, and valid dMarc settings.
2. Check if our firewall block the messages.
3. Since you didn’t find any error on our side via message trace, you could let the admin from army.mil check if our message hit their mail system. Also check if there is any policy or rule block the message in firewall. If yes, they can add our domain into their allow list.