Hi,
As Amit suggested, you can use the message tracking log to see the detailed information of sender, recipients.
Here is the result for your reference.

Note: Through the message tracking log, we can't determine whether the failed messages are due to no-existence recipients email addresses. For example, the above
10test5@cu1.com is a no-existence email address, but
10test1@cu1.com is an existing email address.
Hope my clarification is helpful to you.
Best regards,
Amy Wang
TechNet Community Support
