Since this error is happening on a Mimecast server that's outside Office 365, you'll need to check a few things to fix the ...

Since this error is happening on a Mimecast server that's outside Office 365, you'll need to check a few things to fix the problem.

Check the NDR for information about how to fix the problem. The NDR should tell you what went wrong and how to fix it. For example, the sender should check the spelling of the recipient address, or try to clear the recipient's entry from the Auto-Complete List then resend the message.
Contact the recipient's email admin. Provide them with the error and the name of the external server that reported the error. You can find this in the NDR. While the error refers to a mail loop, the most likely cause for the error is that the recipient doesn't exist in the receiving organization's Mimecast hosted email address directory. Suggest they perform a directory synchronization between their on-premises directory and their Mimecast directory.

This issue is outside Office 365. If the sender can't fix it by resending the message to the correct email address, then only the recipient's email admin can fix this. Unfortunately, Office 365 support can't fix these kinds of errors.