The NO-DOMAIN error often occurs because Trend Micro Email Security checks the original sender of the email. If the sender is not registered in Trend Micro Email Security, the email will be denied because the Trend Micro Email Security Relays are not open and only accept emails from registered senders for security reasons.

The error can also happen under the following scenarios:

• Scenario 1: When an external sender sends an email to a registered Trend Micro Email Security user/domain and you forward it to another external user/domain.
• Scenario 2: When an external sender sends an email to a registered Trend Micro Email Security distribution list (DL) that includes external users.

As workaround, you can add an outbound rule condition: if the sender is part of the internal organization, it will route the email to Trend Micro Email Security.

You can check the screenshot below for reference: